alt Hacker NewsSpeaking entirely out of my ass here:
FOMO for sure is one of the driving factors.
"We cannot risk looking outdated". So weak management, probably.
But also talent availability I suppose. If there's a new trend, the pool of people you can hire include many that are in on that trend.
UI frameworks too, probably. The modern thing™ does the modern thing™ and you do want to be on the modern thing, because you fear that only that receives security fixes or whatever.
If UIs today still looked exactly the same as Windows 2.0 or System 7 or CDE people will be bored to death. Aesthetics come and go and come back, it's part of how humanity worked for a few centuries already.