Not that it's non-obvious, just didn't stop to think about it until now...
Fact #1: mobile carriers charge customers for airtime.
Fact #2: Google Talk uses data channel to transfer voice.
Corollary: Google Talk jeopardizes carrier's business model, and puts a strain on their data channel capacity. This can't make carriers happy.
Corollary 2: There will be no Google Talk for Android.
So if you're developing your own VOIP application, now is a time to rejoice and have a second breath - the first to get to the market share hits the jackpot.
UPDATE: See the UMA conspiracy theory.