Sometimes this is because the new design is missing features or capabilities compared to the previous version (reddit for example [EDIT: I have been re-educated on this point; I should have chosen a different example here]), or because it introduced changes that are contrary to how the product used to work (such as when Facebook switched from a chronological feed to The Algorithm). But just as often it feels like users just dislike change. The new thing feels wrong because it's unfamiliar, not because it's objectively worse. The Stack Overflow redesign a couple years back drew so much outrage at the time! I hated it too! But now that we're all used to it, if you go back in the archives and look at the previous design... it was worse. The new design was better, it just took some getting used to.
Maybe it's just because I don't habitually run with my browser set full-screen, so I'm not seeing all the "wasted space" people are complaining about, but I gotta say the new design looks a lot better to me aesthetically, and I like having the sticky sidebar available as I'm reading, updating as I scroll to give context to where I am in the doc. (And I also like how it goes away when I reduce the window size, just as it should.)
Not that I'm saying you shouldn't like what you like. I don't envy what designers are up against, I guess is my point.