It fundamentally changed the series tone
While Star Trek always had aspirations to be more than what it was, the shows were largely morality plays with minimalistic sets and CGI budgets. The idea is to use unique set designs and practical effects to create a lived-in universe. But one of the byproducts of this was the show feeling more like a stage play, where CGI was understated and not relied on much.
It had a beautiful DIY feel to it.
Then Discovery came along and essentially turned the show into every other streaming show at the time. Big budget, an overreliance on CGI, and so much green screen that it was nearly inescapable. What practical sets the show did have were draped in green screens to add unnecessary effects to the set.
The show-stopped feeling like a Star Trek show and started feeling like every other paint-by-number science fiction or comic book movie property of the time. Some fans loved this change, as it they took it as a sign that CBS was taking Discovery seriously, and treating it more like how Disney treats Star Wars.
But for a good section of fans, that’s not what they wanted the new era of Trek to be. We loved our little sets and our carpeted bridges. We didn’t want everything to be shiny and new. We wanted what made us fall in love with the franchise in the first place.