This is one of the hardest questions to answer. It's difficult because, for starters, we don't even know that Duncan doesn't want to play center. He's never come out and said those words. It's kind of always just been the rumor ... but it's also backed up with reality. Whoever starts next to Duncan is always refered to as the center. Even that time that Danny Ferry started next to Duncan in the playoffs versus Phoenix, Ferry was the "center"
Even though Duncan has never said it, I'm pretty sure he wants to be a power forward for the rest of his career. This summer when he shrank from 7-foot to 6-foot-11 or whatever was just another indication that he's trying to fit the power forward mold.
As
whottt suggested, I actually do believe that the main reason is Duncan's legacy. He wants to be the best power forward ever. A lot of people regard him as the best ever already. If he were to all of a sudden play center, where would you rank him? It'd be hard to say that he's the best power forward ever when he spent the second half of his career as a center.
So yeah, even though Duncan usually doesn't care about what people think or how he's rated, I do think he wants to remain a power forward for historical reasons. Over the years, obviously, he's pretty much been a center. Look at the playoffs, when Horry is in Duncan is no doubt the center.
The only thing that makes me not 100% certain of this is looking back at the players the Spurs tried to sign over the years. Chris Webber, Jermaine O'Neal and Elton Brand come to mind as players, who if the Spurs signed them, would have to refered to as power forwards. I don't think he could hide the fact that he's a center when he has the 6-foot-5 Brand next to him.
Other reasons don't make too much sense to me. Defensively, perhaps he doesn't want to be pigeon-holed into always being stuck on the big lumbering center. David Robinson, and now Nazr Mohammed and Rasho Nesterovic, often guarded the other team's center. Duncan usually begins the game guarding the other teams power forward. However this isn't always the case when the other team has a superstar power forward. Against the Suns, Duncan wasn't matched up against Amare Stoudemire unless he had to be. Against the Timberwolves, he's not put on Garnett until the fourth quarters.
The role of a power forward and center in the Spurs' defense isn't too different. They both have to rotate and they both are expected to block shots. It's not like the team only directs the opponents to the center. It's just a matter of which bigman is on the strong side of the ball.
Offensively, Duncan is playing pretty much the same and is in the same spots on the court no matter who is in the game. Of course he'll have more room to operate if Horry is on the court than when compared to Nazr, but he's pretty much in the same spots. 4-Down is pretty much the same no matter who is on the court with him.
I can't really think of why it seems Duncan perfers being a power forward other than legacy. I know it doesn't go with his personality or selfless history, but it's the only thing that makes sense. Especially considering the shrinkage in height this summer that puts him more into the "power forward" category.
Or perhaps, it boils down to him enjoying inbounding the ball. Whoever the power forward on the court is inbounds the ball after made baskets by the other team. The center does not. Could that be the answer?
