Let me get this straight, Kemono Friends is painfully bad in quality, the storyboard is also painfully simple and predictable because of its "For children" nature.
Why does such a sloppily-made CGI anime it suddenly exploded in Japan, then?
1. Animal Girls (known as Friends).
First off, Japanese really loves turning all sorts of stuff into girls (Gijinka), as just loved watching them as much. Ranging from young, cute girls (such as the Sand Cat, Otter), to teen girls (such as the protagonist Serval, her Manga exclusive friend Caracal, the two foxes), and older, stronger figures (the Grey Wolf, Lion, Moose). Especially dog/cat girls, the Japanese absolutely loved this stuff.
This became a strong basis for Kemono friends to start with - I mean come on, some of us are perverts and are just here for the cute girls, aren't we?
Because of the nature of the show and the game it's based off, except most felines that wore a white tank-top with a bow tie on their neck, and a dress that matches their real-life patterns and fur colour (with Lion and the game-only Tiger being exceptions), the Friends featured usually wears school uniform-like "fur" - their understanding of clothes as they don't even know they are detachable instead of stuck onto their skin, that matches their colouring and their age, which just makes them even more beloved by the Japanese. This also prevented them from being extra revealing, unlike the meta where girls are super booby and sexy and/or wear something that can barely be called as clothes, unless they are conditioned so, such as the Otter who wore full-body swimsuits instead because she is accustomed to water. (Probably with the exception of Emperor Penguin, but she's a penguin, afterall......)
2. Serval, along with her friendship with Kaban-chan, and some other small details that REALLY suited Japanese's taste on the whole.
We all wanted a cute girl that is energetic, ever-trusting, sees everything you do as if it is something incredible, brainlessly dumb yet brave like a never-ending sunshine, don't we (at least for the Japs, again)? Partnering her with a shy, powerless but smart girl, and you get one hell of a moe combo.
Fun-fact: One of the reasons why Kemono Friends exploded is its cute OP. Not only it is voiced by a few good VAs and has a very good, lightheartly melody, it's about "Welcome to Japari Park anytime!","No need to pretend, feel free to be here!", which is EXACTLY why the Japanese are pretty insane - They always have to maintain polite even when they wanted to throw punches into someone's face, heavily staged-society where you need to be 100% obedient, pressuring lives, etc., and Kemono Friends' apocalyptic yet paradise-like world where problematic people are gone, while only animal girls that always welcomed your arrival is just what they needed in such a crapsack world of theirs.
Because of the very different culture between the East and the West, it is understandable why it is kinda crazy in Japan and sometimes China, it doesn't work very well in the West especially when the things of interest is very different.
3. Anti-meta storyboards that somehow worked, and the effort put into the story despite its very limited budget.
Sometimes, instead of killing everybody, with the remaining goes totally bonkers, the world threw into absolute chaos, all we need is just something we can truly love endearingly without screwing everyone's brain over with some over-the-top philosophy. (I'm looking at you Evangelion and all those chouuni animes out there)
We had enough stuff in our real life that we'd absolutely wanted to just go "screw everyone", and while we can't deny some of the more serious toned story such as Gundam are so well-written and awesome, we already had more than enough to bear in our lives and the least we wanted is someone you loved is slapped into a bloody mess (which is the reason why there is almost an outrage when something bad happened to Kaban-chan during late story), but wanted some cute girls doing some cute stuff instead.
Japanese' tough, cramped society life, in fact, helps more than enough in terms of this - Not everyone wanted to put a load of stress by seeing others die painfully, and some are already sick of the booby bounces and maid dresses other animes are providing. Because of the extremely bad CGI, the lack of obvious fan-servicing actually avoided this is a fabulous manner.
The story is painfully normal for a children's story, but you will find some small details that make people wonder quite a bit, which is the main reason why Kemono Friends suddenly exploded in popularity, as well as showing how much small-but-noticable details are put despite the poor budget the anime is given with. For people who don't know about its background, before Kemono Friends exploded in Japan, someone who watched the anime somehow analyzed the out-of-pace ED of the Anime, and found out that the seemingly random photos are in fact pictures of real-life abandoned theme parks. Given that this story is based on a place called Japari Park, people are like, what the absolute hell? Which caused it to gain massive popularity afterwards.
There are also numerous small details that many have created theories to try and explain everything, or just otherwise are unexpected and gives more flavor to the story. For example, Serval's famous pendulum swing which became a meme is in fact what a real-life serval cat does to compensate the lack of 3D vision, and much more.
4. It's somehow so addicting to watch.
<Above image sums what's now with my mind.
Really, once you get with it, this anime really gets into your head and you don't really know why. And no, I am not high on drugs.