Hmmm, let's see:
AT&T's customer service is incompetent. A few years ago, I had to call their customer service and speak to "managers" every month for six months to get them to remove a recurring monthly charge on my bill. Everyone I spoke to at AT&T was supposedly clueless as to what it was or why it was on my bill. I wonder how many other people have charges buried deep in their AT&T bills that they've been paying for years like a special AT&T "tax"?
Sprint's customer service sucks and their network blows, as well.
T-Mobile's network is far from covering the majority of this country. Not talking about their LTE network -- just their network in general. In many parts of the country, you'll only get T-Mobile coverage within 10 miles of an interstate. Try traveling with that kind of coverage. It's not a pleasant experience.
Who else is left but Verizon if you're looking for a nationwide (not regional) carrier? Sure, Verizon's far from perfect; but IMO, they're the lesser of four evils.