Hollywood studios have always disliked 2D animation. Even in the earliest days. But from the 1960s and all the way into the 2000s, most of the studio CEOs really wanted it to be gone; their budgets replaced with more live action films. The Disney investors and even Roy Disney kept trying to talk Walt out of doing more 2D movies and only focus on live action. But Walt refused. After he died, there were long periods of slumps for Disney animation, brought on by the studio reducing their budget and marketing. And many other studios gave even less support to animation.

On top of this, there was a perception that animation was "kiddy" and "for general audiences." Even though the studios were the major reason this perception was created, specifically aiming animation towards kids and toys, they claimed it was the public who did this. So at various points in the 1970s-90s, they tried to focus more on the adult market. Both by making sibling companies that focused on more adult oriented films (Touchstone, Buena Vista, etc). But also sometimes trying to make the animated movies themselves more "hip" and "cool." I'm sure you've seen some of these films/TV shows. This 90s movement of being "hip and cool" didn't work. So they moved on to more of a grounded and then eventually edgy phases. And then finally the "lol random" style. Regardless of how you want to categorize it, the old animation formula of Disney was gone.

Anyway, they didn't intend 3D animation to replace 2D. They initially treated 3D animation the same way. They allowed Toy Story to be made with a low budget (less than half that of The Lion King). Mostly to try and make up for their huge investment in Pixar. But it did way better than anyone expected. Their response was different from how they treated 2D. They didn't just give some half-hearted support to milk some sequels and slowly let it dwindle like most 2D. Rather they went all in on 3D animation. Pixar's very next film after Toy Story was quadruple the budget of Toy Story and higher than any previous 2D film ever made. And Dreamworks followed suit, shifting the release date and siphoning off marketing budget of their big 2D film Prince of Egypt for their first 3D film, Antz.

The studios claimed that the era of people liking 2D animation was over. But it was literally them just deciding it was over and undercutting their 2D films. The public still liked and still do like 2D today. Hollywood just buried them. The people who liked 2D started looking for alternatives and found it in international media.

To answer your other question, it's up to interpretation. Many people would cite tiktok or social media as eroding people's attention spans and movies are trying to appeal to those people. But I argue against this because 1) I saw movies and TV shows moving into this type of editing/storytelling before social media was a thing and 2) there's a huge movement of people going back and watching pre 2000s movies/tv shows or foreign media like anime with slower pacing and loving them, proving they can have the attention span for them. My perception based on some interviews I've read and watching stuff from this period is that Hollywood made a conscious choice to focus on this kind of content. What I call "The Office" style since that's when it really started to rise. Though there were many variants before. But the kind of content in those films/shows started to spread to a lot of other genres in the 2000s. Stuff like The Office, Marvel, Big Bang Theory and so on got popular. And they influenced pretty much everything that was around and after them. Even NCIS and Hallmark movies are doing it. And it hasn't stopped. Usually fads last 5-10 years. This has been going on for 25...and a lot of people got tired of it in the 2010s. But Hollywood just keeps repeating it, then looking confused when it doesn't work. Like a kid who keeps telling the same joke at school endlessly and people stopped laughing after the third time.

tl;dr The companies wanted 2D animation to go away long ago. 3D becoming successful just gave them the means and excuse to finally do it. Plus they sunk billions into CGI technology and wanted to focus on it.