Because AT&T has the leverage in the relationship with Nokia and Microsoft. They're going to do what's in their own best interest and that usually means being against the best interests of the consumer.
I think they simple don't care because it's not a top selling phone and it's like not a profitable platform for them. But it's also possible they're hoping to push a few users to their newer phones already running Amber.
Is it the end of the world for us? Of course not, but it's the principle of the matter that irks me. It's obvious they can't be bothered and they do have a history of hostile business practices against their consumers.