In the 2000s, you started seeing popular sci-fi books (then movies in 2010s) like The Hunger Games and Maze Runner that depict teenagers as the opposed class.
Personally I think the 2000 Japanese classic Battle Royale started it all.
The mid 2000s dystopia fest in young adult stuff is just follow the leader with Hunger Games. The oppression is used not for it's own sake, but because its an easy excuse for your teens to be more "mature" and more in control of their own lives. It's escapism for teens who think parents "just don't get us"
I think the difference now is that happy endings used to be the norm, whereas now they are not necessarily there, or the protagonists are pushed through such extreme losses that the win ends up looking pyrrhic.
This might simply be a function of the amount of productions - in the history of mankind, we've never seen so many new stories circulating so widely on so many different media. Smart authors want to look different and "real", so they might veer dark more liberally than they used (or rather were allowed, by public and editors) to.