Also, 1080p (with 3D this time) cameras are much less forgiving of age than the videotape technology sued during the previous reunions. We could have accepted Troughton's more-craggy face or Pertwee's increasingly poofy hair, but there's NO WAY anyone would accept any of the classic Doctors in a realistic context (save McGann - that guy is the most recent, and apparently the man just doesn't age). Even the explanation in Time Crash only got us so far, and my own desire to see the older guys in action again is weighed against the practical needs of seeing a story that makes thematic and contextual sense.
Time Crash supplies a total solution.
Also, it's not just McGann who doesn't look much older. Davison and McCoy both look close enough.