Yeah, I think the main thing is just the rise of the three point shot.
As more and more people shoot more and more threes, there isn't as much defensive value in a guy that's tall but can't hold his ground or move well and there isn't as much offensive value in guys that can get shots in the paint unless they're extremely good at them.
So you either need to be able to shoot from distance or absolutely dominate in the paint.
The next level of it is guys that are 6-8, 6-9 used to be power forwards or centers and play in the paint, but now those guys from a young age are shooting threes and handling the ball and I think are generally more skilled than they used to be so they're harder to guard for your classic big man.
And getting into a little bit of complete speculation, I'd guess that most players are more athletic than they used to be which helps narrow the gap when going up for rebounds.