From what I know, the reason Saint Seiya got canceled after the Poseidon arc was less spectacular and was something that could be called "the humdrum of life," heh.
The Asgard arc is one of the main reasons. Even though it's popular outside of Japan (although I wasn't a huge fan of it when I was a kid, lol), it was not popular in Japan back then. It must have been disappointing for many people working on it that a filler arc with relatively high production quality and care, designed to feel like more than just a typical filler, didn't resonate with the local audience. This led to a decline in viewership and popularity that surely affected the merchandise's condition in a negative way - a clear sign of trouble for the franchise.
It's not that the manga sales went dramatically downhill, however. The manga sold fairly well back then, but the Poseidon arc was simply way less popular than the iconic Sanctuary arc. Combined with the anime's decreasing popularity and the fact that the show itself had outpaced the manga, it led to a serious discussion about putting the series on a hiatus or even canceling the anime entirely after the Poseidon arc. The studio was faced with a tough choice: continue the anime with another filler arc, risking a further decrease in popularity regardless of the quality, or take a safer route by trying something else, like focusing on a different and more popular franchise.
This is where Dragon Ball Z comes in. DBZ was a big hit and its popularity was gradually increasing, not decreasing like in Saint Seiya's case. DBZ also faced a similar situation with the anime outpacing the manga at one point. And yet, those filler moments, like the ones that happened during the intense Goku vs. Frieza fight, didn't negatively affect viewership, even if they made the whole fight dragged out and objectively less dynamic. As time passed and more Dragon Ball manga chapters were released and ready to be adapted, the Saint Seiya anime had kinda become obsolete and had to move along to make way for the new hit series. That meant the Saint Seiya anime entering an indefinite hiatus. It also affected the movie franchise, as the last movie, Saint Seiya: Warriors of the Final Holy Battle, was released in the same year the TV series ended, and no new films were produced until 2004's Saint Seiya: Heaven Chapter - Overture.
That being said, it's not like the Saint Seiya anime was forgotten. There were concrete plans to adapt the Hades arc in the 1990s, most likely in an OVA format. An official announcement in V-Jump signaled the possibility of the anime's continuation, and in 1990, a drama album dedicated to the Hades arc was released with high-quality tracks, including the famous "Dead or Dead" tune, which is widely believed to be the planned opening theme for the Hades OVA. The existence of such an announcement and professional pre-production material surely indicated that the first steps were taken to make the Hades arc a reality. And yet, it didn't happen. Many fans, including me, blame the crash from the early 1990s (the collapse of the "Bubble Economy" in Japan) for negatively affecting the production and eventually canceling the whole project. Personally, though, I don't blame anyone for it. It would have been difficult to produce an anime in such tough circumstances, especially when the company's focus had clearly shifted to another popular series (DBZ). Reviving a series from a long hiatus would have been a huge gamble. Luckily, the topic of this arc's anime adaptation returned in the 2000s and resulted in it being entirely adapted into the OVA format.
Anyway, to answer your question
@tchitchouan, the Saint Seiya anime was not canceled because of DBZ in my opinion. DBZ's success simply made it a decision to take a longer break from the Saint Seiya anime, or even step away from continuing its story in the animated format, easier. It wasn't the cause, but rather solidified a decision that had already been made.