T-Mobile only started up their 3G network this year (and the 2nd half of the year at that). So that's why there are so few phones for their 3G bands.
I'm not sure we'll see a wide selection of unlocked T-Mo 3G US phones in the future though, given that T-Mobile is the 4th largest US carrier. They have approx. 30 million subscribers while AT&T has approx. 75 million. So they economic incentive for companies like HTC, Nokia and Sony Ericsson to make unlocked phones on their bands is obviously less strong that for ATT. The pressure may largely be on T-Mobile to pick up some really cool 3G phones, rather than relying on unlocked imports to keep their customers happy.