If you use large amounts of mobile data, their plans very quickly become obnoxious (throttled after 15GB) and expensive ($80 plus taxes/fees).
Compare that with T-Mobile, who offers $70 a month (including taxes/fees) with throttling at 50GB and 20GB of hotspot data for $15/mo more, or even AT&T or Verizon, and it's more expensive for less service.