why can't I upgrade to the nexus one on their site but when I go to T-mobile

[Lauren]

site I'm able for an upgrade? I go to the google.com/phone site and I fill out everything but it says I'm not eligible for an upgrade but if I go to T-mobile.com and sign into my account I'm eligible for a full upgrade for the phones on their site but the Nexus One is not on their site but thats the phone I want