[quote=Magic Magnum]
Granted that does explain Rose, Amy and Donna.But I'm thinking about Mickey, Jack, Martha. They were still around, hell they were all still pretty involved in the whole field of aliens and time but the the switch of David Tenant to Matt Smith still felt the need for one last goodbye as if after the regeneration he couldn't see them again.I'd include Sarah Jane, but there's an episode in her own series where Matt Smith shows up, so obviously he's still able to run into her.
[/quote]

Eh, I guess Mickey and Martha just weren't as popular. Though you're right that there isn't much of an excuse for Jack. 

Although, I did hear an amusing theory that the reason why Jack wasn't around at all during Matt Smith's run is because Moffat can't kill him. :hehe

Hopefully he'll show up during Capaldi's run. I really have no basis for this theory; it's just a hope.