Seriously. Nathan Lane, Ernie Sabella, Matthew Broderick, Moira Kelly, Whoopi Goldberg, and Cheech Marin? DisneyToon sequels usually don't have that many original cast members, especially when they're such big names.
Possibly simply because of the name it was associated with. The Lion King has probably been Disney's biggest movie in the last fifteen years at least (not counting Pixar). It probably didn't take much to get the actors to come back.
Besides, the very nature of the film meant that even seemingly central characters like Simba didn't really get that many lines. Most of the original cast that returned were probably able to record all their lines in a day or two.
That doesn't apply to Timon and Pumbaa, but I don't think Nathan Lane or Ernie Sabella would have been happy having anyone else play those parts. In so many ways, Timon and Pumbaa are based on those two. The charisma and chemistry of those characters comes from Nathan and Ernie's real life friendship and respective personalities. It'd be tough to get that to work so well with other actors. Plus considering this film stars those two characters, it'd be just so wrong to have different voices.
I still think it was ridiculous having Edward Hibbert playing Zazu in this one and TLK II when Rowan Atkinson put so much of his sarcastic Blackadder esque personality into the character in the original. Especially considering that it'd be possible to recite all Zazu's lines from the second and third films in about thirty seconds.
That's getting off topic though.
The most predictable thing about the bush is that the bush is unpredictable
It was a sequel to the most popular Disney movie ever, I think they would feel obliged to do it. Plus none of them had that many lines to say, so it could probably be done in a day. And then they would go on with their daily live.