Because when ordering T-Mobile iPhone from Apple store, you do not have to input any account information to verify one as a T-Mobile customer. When ordering an AT&T phone, it makes you input phone no, last 4 of social security, and then the AT&T account passcode. This is just one extra step in the process with 2 additional screens to get through as compared with a T Mobile phone. Ordering a T-Mobile iPhone will be quicker during pre-orders when time is of the essence and every second will count to bagging that Nov 3rd delivery date.