Millenials got so completely fucked by 2008 crash and many of them eventually saw that there's really no way out and things are perma fucked.
But instead of getting demoralized and giving up to apathy, they chose to regress to a time they were happy in the 90s and early 2000s.
It's like a person becoming a simple minded child after a traumatic event to escape reality.
Zoomers would likely do the same if they could, but they can't because they never experienced any of that overly happy Disney era, when things were hopeful to a point of naivety.