I know that some postpaid plans require a credit check. No credit check plans are on limited plans and are prepaid. The only other thing I can think of is that the 55-plus plan does not offer to pay off any devices from another carrier when switching. Also you cannot bring your own phone if it is not paid off and because T-Mobile doesn't pay off those phones that you would have to have another set of phones. Without a credit check most devices are full price. Again I can see a credit check being needed on postpaid plans and anytime you switch over from another carrier where you are not able to bring your own device.
Also keep in mind that that deposit is returned after a year of on time never late payments. I'm willing to bet that whatever they are paying on AT&T they would make up at TMobile and cover that deposit in two months time. And then 10 months later get it all back.
No credit check is post paid, but you lose out on some features.
the 55+ is a credit check only plan.