It's because T-Mobile isn't focused on building and sustaining incredible customer experiences. T-Mo's mission is to rapid fire defend and attack the other huge carriers like AT&T and Verizon. By attack I mean everytime they have a new "UnCarrier" promotion and by defend I mean everytime they have a deal such as the $5 a month iPhone deal, family plans that are advertised on TV and so on. That's why so many of these new promotions are confusing, hard to figure out and half the time the employees don't even know what's going on outside of a pre-compiled instruction sheet they're sent.
To T-Mo's credit, a lot of what they do helps out existing customers. I don't think I'll switch from T-Mobile anytime soon. The only downfall is that each new 'foot in the door' deal often come at the expense of UX polish.
I don't even know if that makes sense, sorry, I'm super tired!