Jim Alling, CEO of T-Mobile explained to the press at a press conference in Barcelona, Spain:
"Make no mistake about it: We would love to carry the iPhone. However, we want the economies to be right for us."
Boom. See Apple? Not everyone wants to sign a four year, $15.5 bullion deal with you (that's how much Sprint paid in 2011). That said, there are apparantly over 1.5 million users on T-Mobile's network using unlocked iPhones, so certainly there's some demand there. But since T-Mobile can offer these users with a SIM card for an unlocked iPhone, why offer the phone? With Android taking a far greater market share, it looks like carriers can carry on just fine without the iPhone.