Why doesn't T-Mobile sell iPhones as soon as they get released?

I think one of the main reasons they are behind is because they take a month or two to start offering the new iPhone model to their customers so people flock to Verizon, Sprint, and AT&T

T-Mobile doesn't have to sell you the iPhone directly from them
Remember that T-Mobile doesn't do contracts so you buy the phone at full price and its actually the opposite, more customers are coming to T-Mobile because who doesn't love unlimited data

Because the phones don't just magically appear as soon as they're announced. They have to be manufactured, programmed & shipped. Did you think that Santa just conjured them up out of thin air?