He just wasn't all that efficient. His career TS% is just .551, which is nice, but it's not that far above league average. His best season he shot .579 (to put that in perspective, that's what Malone averaged).
So the reason I say he's overrated as a scorer is because he has all these post moves, size/length advantages, and supposedly the best fundamentals so people fawn all over him, but the reality is Dwight Howard with almost no moves can score 20 and shoot a TS% of .600+* so where are all those moves really getting TD?
*And consider Howard's FT% is bringing that down, while TD's is bringing his up