What causes popular anime to become forgotten?

Never heard it about it, been watching anime for more than 20 years.
It's pretty simple, Popular has like two tiers, There's popular holy shit good shows trending for the recent and past years and then there's the Masterpiece culture type. So that type is like shows that really came with a bang, they were all over japan (like u cant turn to a direction in tokyo without seeing posters, merch sell, promos of it), they set the standards for other upcoming animes in that genre, and also they had an immediate impact in the western world, like that was your go to see anime for every fan or new anime viewers in the west introduced to japan's culture. that's the difference of being forgotten or not.

I'll give some examples: let's start with the biggest once, has to be one piece, like it's all over world, netflix adaptions, stage plays like i can go on and on, when people think anime, One piece jumps to their head and it's huge huge culture thing in japan, i can't even call it anime there.
Next one i'll go with dragon ball Z, it set the standards for so many anime shows in that era, it was so popular that all western countries channels got it, personally it was my first anime, it's not my most favorite now anymore because i was exposed so many others, but i grew up watching it and without it i doubt i knew what anime is.
Other examples can be Inuyasha, romance and shonen classic, but i think what it does more for the western world is that it shows the Japan's old time, which looks completely different to our history, first encounter with a Shrine's Mikou, how people dressed there, the crazy haircuts, lots of stuff, so it was very educational.
Sakura the card capture perhaps for many kids and mainly girls.
Another one is Ranma, is set the standard for so many Romcoms and was huge all across the world.
(I wonder if Frieren can be a next one)
And this is my wish, hopefully Re:zero it's my fav anime so my comment is bias here, but i think it did set the standard for many Isekais, wasn't it the first one animated? and it was a culture thing after the first season, not sure if it's the same case today in japan tho.
Lastly, not much of an anime, but there's not one person who reads manga and doesn't know Berserk. And you literally have Dark souls/Elden Ring games taking refences from it.

But ya then i can mention a lot of other super popular good shows with many movies and OVAS that WILL BE FORGOTTEN with time, like Haiikyu, My hero academia, Euphonium, Spice and wolf, Psycho pass, i can go on to mention more and more, like Jujutsu Kaisen and Demon Slayer (even tho im not sure where to put it, yes its popular all over the world, but maybe it's too early yet and i can't tell if it'll be forgotten 20-30 years later, most people love it due to insane animations, i like it, but it's not on my top anime list, i'm more of a story person and it's kinda Meh there)

Hope This answers your question, the difference is how impact it has/had all over the world, if it setting the standards for a new era of anime and how it inserted to Japan's current culture.

"What causes popular anime to become forgotten?"

1. Anime fandom working on hype, instead of actual appreciation.
2. Anime fandom being focused on social experience and social interactions instead of did they actually liked said anime or are they just using it to talk with people.

And it's unclear to my why stuff like Naruto and JoJo have a bigger staying power that lets say Nana (none of those three is a favorite of mine), but the anime society somehow decides and some stuff like Noragami is remembered despite being aggressively boring and masterpieces like Uchouten Kazoku are forgotten.

In the case of Haruhi Suzumiya, there are way more stuff left in the source material that need to be animated but for some reason KyoAni and other studios decided to not pick them up. Popularity will stay if there were more adaptations from the leftovers of the source material. But sadly this never happened. So this is one of the reason why Haruhi franchise has died. The novels themselves is now on hiatus, only one or two other novels were released few years back. So there aren't enough stuff to keep the franchise going.

As for anime in general, the hype dies very quickly among the typical anime fandom - the ones that are more into mainstream stuff than what's niche. So in a few years, unless some of these fans constantly talking about them, these mainstream anime shows will be forgotten as time passes.

Bad endings, gradual decline, lack of complimentary media after the manga/anime ends like video games or promotional ads or collabs.

Never heard it about it, been watching anime for more than 20 years.
It's pretty simple, Popular has like two tiers, There's popular holy shit good shows trending for the recent and past years and then there's the Masterpiece culture type. So that type is like shows that really came with a bang, they were all over japan (like u cant turn to a direction in tokyo without seeing posters, merch sell, promos of it), they set the standards for other upcoming animes in that genre, and also they had an immediate impact in the western world, like that was your go to see anime for every fan or new anime viewers in the west introduced to japan's culture. that's the difference of being forgotten or not.

I'll give some examples: let's start with the biggest once, has to be one piece, like it's all over world, netflix adaptions, stage plays like i can go on and on, when people think anime, One piece jumps to their head and it's huge huge culture thing in japan, i can't even call it anime there.
Next one i'll go with dragon ball Z, it set the standards for so many anime shows in that era, it was so popular that all western countries channels got it, personally it was my first anime, it's not my most favorite now anymore because i was exposed so many others, but i grew up watching it and without it i doubt i knew what anime is.
Other examples can be Inuyasha, romance and shonen classic, but i think what it does more for the western world is that it shows the Japan's old time, which looks completely different to our history, first encounter with a Shrine's Mikou, how people dressed there, the crazy haircuts, lots of stuff, so it was very educational.
Sakura the card capture perhaps for many kids and mainly girls.
Another one is Ranma, is set the standard for so many Romcoms and was huge all across the world.
(I wonder if Frieren can be a next one)
And this is my wish, hopefully Re:zero it's my fav anime so my comment is bias here, but i think it did set the standard for many Isekais, wasn't it the first one animated? and it was a culture thing after the first season, not sure if it's the same case today in japan tho.
Lastly, not much of an anime, but there's not one person who reads manga and doesn't know Berserk. And you literally have Dark souls/Elden Ring games taking refences from it.

But ya then i can mention a lot of other super popular good shows with many movies and OVAS that WILL BE FORGOTTEN with time, like Haiikyu, My hero academia, Euphonium, Spice and wolf, Psycho pass, i can go on to mention more and more, like Jujutsu Kaisen and Demon Slayer (even tho im not sure where to put it, yes its popular all over the world, but maybe it's too early yet and i can't tell if it'll be forgotten 20-30 years later, most people love it due to insane animations, i like it, but it's not on my top anime list, i'm more of a story person and it's kinda Meh there)

Hope This answers your question, the difference is how impact it has/had all over the world, if it setting the standards for a new era of anime and how it inserted to Japan's current culture.

@Ron014 Only problem here, none of your examples have been FORGOTTEN. Those are all titles people STILL love and talk about. Now try something like Blood + maybe. Might be more in line with the theme of the Post.

Time passing means new people getting into anime fandoms, older fans getting out of it (or at least lowering their involvement), so 'old' anime (anything who aired more than 10-15 years ago really) quickly get forgotten (or rather it's not even been introduced to newer generations in the first place):
I often talked about how shows like Mayo Chiki, Haganai, D-Frag and Witchcraft Works used to be recommended to me on every corner of the internet at the goats of comedy but nowadays most new people don't seem to have ever heard of them (for some reason one of these shows is not even tagged as a comedy on mal, which also doesn't help).

I think some older anime are also very much a product of their time (like those looking at or making fun of the otaku/game culture of their time) and are not going to be super interesting to newer generations for that reason ('why I would I watch a show making fun of the tropes of 15 years ago when I don't even know those tropes?' you can't blame them).

Why are none of the comments addressing this 😭😭 Am I crazy??

@DeadBored
Do you want the explanation?

@thewiru Is it somehow related to the show? (I haven't seen it btw)