You should instead be asking why this angular design was popular at all in the first place.
If you look at pre-70s anime, it's all cute round characters. It was Leiji Matsumoto's Yamato series and it's success that spawned the "80s look" where everything was done with angular shapes and stylish hair to denote passionate characters, etc. It's what Shimamoto famously drew upon when making Blazing Transfer Student and later the anime shop clerk from Lucky Star, if you remember.
It was simply the style at the time, and eventually other styles subplanted it along with a digital revolution that made animating much different than how it used to be. Back in the 90s the old way got really stuffy, with the insistence on dark shading and detailed frames making it difficult to animate well, so the digital revolution really brought life back into the industry.
So to answer your question, I think in fact, animation only reverted back to the pre-Leiji style with cute round shapes moving fluidly like you see in Mahoutsukai Sally or Tetsuwan Atom or Tetsujin 28-gou. And it was accented by the themes of the time, like moe in the 00s, the shapes of the 2010s isekai boom, and now a revert back to detailed shading with simple shapes that you see now.