Except Sprint has the crappiest coverage on the West Coast
Verizon has the best coverage out here, AT&T isn't all that much worse (despite the exaggerated whining you hear from some people). Unfortunately, Verizon is also NOT GSM, and they love to cripple their phones, as well as nickel and dime you to death. It took how many years before they offered Bluetooth phones? But if you want to sync to your computer with BT, you have to pay extra!
T-Mobile just doesn't have the nation-wide coverage right now.
So, AT&T ends up being the lesser of all evils.