I think that's because if a player gets lots of appearences for the U21s they're not good enough to make the step up. The best ones nearly always breakthrough to the full squad early and hence aren't capped by the U21s up until they're 22/23. For example there is a lot of talk that Wilshere has been left out of the U21s because Capello wants him for the full team, and he's only what, 18?