I blame AAU and guys coming out of college too early, the average rookie age of Robinson, Hakeem, Wilt, Duncan, Ewing and Kareem was almost 23 years old. I left out Shaq because he came out as a Sophomore, but even he was 20 as a rookie. These guys all got a lot of playing time and had their bodies mature as their games developed, they got to see what works and what doesn't in the post. I don't think it's any coincidence that the last great post player also happens to be someone that went to college for 4 years.
It's funny how a lot of the guys that came out of HS or entered the draft as freshmen almost all seem to have their breakout years around 21-23 years old, basically the time they'd be entering the league had they stayed 4 years. I like Andre Drummond a lot, but he's a perfect example of everything at work, on the AAU circuit he wanted to play like a SF and never really focused on developing a back to the basket game, despite having every tool you could think of to be a dominant post player. We can see how naturally good he is at the game right now in the NBA, imagine if he stayed 4 years at Uconn and learned how to play with his back to the basket just off trial and error of moves against guys smaller than himself. By his 2nd or 3rd year the entire offense @ Uconn would have revolved around him and he would have gained confidence in his offense. I mean, someone like Hibbert wouldn't have even been drafted had he tried to come out as a Fresh/Soph.
I maintain that the next great center will either come from Europe, or he'll be one of those undersized PF's that has a late growth spurt and shoots up to 7"0.