In my case, the simple fact that it can't run Verizon. They are hands down the favorite service around here, best coverage, simple as that. My AT&T is spotty, relatively speaking. More than 1/2 the time I have no coverage at home. However, I seldom engage in extra-home telephone activity, so I live with it. T-Mobile is no better here, apparently.
Do a local survey among random smartphone users in my area and you'll come up with 80+% Verizon, and that's probably conservative. I have stopped asking.
I was with Verizon for many years before moving to ATT with this phone. I've always heard and believed that Verizon has the best coverage but to be honest, I have yet to experience an inability to make or take a call due to location or have and quality issues with a call. No doubt I would if I went far enough out in the country but for now all is good.