Unlike the iPhone that started off its life exclusive to AT&T, it was a mistake to have Nokia to place its Lumia 800 and 900 with AT&T only. Look at the sucess Samsung has with their Galaxy series of phones. By offering their phones with ALL four carriers, its managed to become the #2 smartphone in the sales charts here in the US.
Being an T-Mobile customer, I wanted to try the 800 or the 900 phones, but neither of them support T-Mobiles radio frequencies and your stuck downloading content at 2G speeds.
You lost a customer, Nokia. And no, I don't care for the 720:
A lot of that had to do with carrier interest. Most carriers opted to wait for WP8(Apollo) to support any new hardware running Windows Phone. Verizon opted to not support it due to lack of LTE support. Sprint said they will wait for Apollo as well. Now, that MS has actually made it possible to support LTE, Verizon is now saying that they will wait for Apollo, too.
So you see, it's really more about carrier participation than OEM lack of development.