The new UI is hard to look at.
Vast amounts of wasted space, everything is far too big, the search function has less straightforward functionality than before(like now it automatically sorts by endorsements instead of date published, why would you change this?), and to top it all off it's laggy and has considerably worse performance than the old UI.
I've read a lot of forum posts, reddit posts etc about people talking about the new UI and it's very difficult to find people even being moderately positive about the change. Why would you keep something like this that is very clearly disliked by the majority of your community and users? Especially when its performance is noticeably worse.
At least give an option for people to use the old UI, like reddit does with old reddit. Even if you don't want to "support" the old UI, it'd be nice just to have the option. Especially considering the old UI runs better.