I think - and please don't quote me on this - that for whatever reason, he wasn't interested in being in the movie. Plus, they couldn't have known way back in 2001 that they would need him again, JKR may not have known she'd be writing flashbacks, so he might not have had a contract.

I'll look it up to see if I'm right on him refusing (or at least expressing disinterest.)

ETA: Whoops, no, sorry Christian! Turns out the opposite was true - he was interested, but the director felt 29 was too old. I sort of see their point, but plenty of movies have older actors and actresses playing younger roles.