He's pretty much maxed out his body, he's always going to be lean, and struggles physically in the paint against bigger men.
The other defensive centers are simply much bigger, Noel is listed at 6'11 228, which looks about right
Rudy Gobert 7'1 245
DeAndre Jordan 6'11 265
Pau Gasol 7'0 250
Andre Drumman 6'11 279
Hassan Whiteside 7'0 265
LaMarcus Aldridge 6'11 245
Tim Duncan 6'11 250
Ian Mahinmi 6'11 250
Al Horford 6'10 245
So Noel is really a role player, he has no offensive skills (shoots 30% from 2-15 feet) other than uncontested dunks, he lacks the bulk to matchup in the paint against big men, struggles to box out, he's best on the open floor and with help defense. The closest match is Bismark Biyombo, who isn't as good as Noel in most areas but is a better shot blocker. For a playoff team, Noel would be a 20-25 minute guy off the bench as a defensive stopper and center for the small lineup.