People's nostalgia for the 90s is really because it was the so-called "end of history" era, where the USSR fell and the US had no real enemies and everything seemed to be looking better. Then 9/11 happened and you had the Iraq war and then 2008. It's only in retrospect after that time that people think "hey, the 90s were actually pretty idyllic and peaceful." Hindsight is 20/20, etc etc.

As for why zoomers are now into the 90s, I think there is some picked up nostalgia from millennials around the 90s by osmosis, kind of like how there was some nostalgia for the 70s in the early 00s, limited as it was for millennials, which I think was about the time the boomers first had their midlife crisis (fits somewhat for now I think).