I would say that from 2006 and earlier, designers sought to go "pure CSS" primarily because of the possibility of javascript being disabled on the client. Especially for search engine crawlers at the time, this would be a bad thing.
Things have changed and more and more people run with javascript enabled, and more and more website stakeholders are willing to put up with things being broken for users who don't have it enabled. I know that's a contentious statement, but it's true. If Google Analytics and ads aren't being pushed down to you because javascript is disabled, most site owners are willing to put up with the experience being broken for you too. I'll sidestep the debate of graceful degradation.
Now however, there are performance reasons for going "pure CSS". A lot of CSS transforms and animations are hardware accelerated and/or just animate more fluidly than they would if they were animated via javascript. Javascript is single threaded, and when a webpage is doing a ton of varied activities, it's hard to ensure fluid animation when the only tools at your disposal for drawing frames are pretty crude methods like setTimeout.