No comments why people prefer AT&T?
HSPDA/HSPA+ speeds routinely give 2x to 5x the speed of Verizon or Sprint, even on V/S's best days. Also AT&T gives simultaneous voice/data, which personally I don't find a big deal on the phone itself but it's great with tethering.
AT&T's rates are generally a little better than Verizon's, and some nice features like family plan unlimited text + unlimited calls to any carrier's mobile for $30 (covers all phones on the plan).
The way I see it, Verizon has overall the best coverage area. If everything else were equal, then Verizon would be the only logical carrier to use. Of course it's not all equal.
AT&T has better rates than Verizon, and with iPhone specifically AT&T offers better performance & features. AT&T's coverage area is "very good" - while there are some who choose Verizon because they don't get a good AT&T signal where they live, in terms of percentages it's a small number of users. The price & particularly performance are significantly better on AT&T vs. Verizon, so that accounts for the bulk of iPhone users choosing AT&T.
Sprint has unlimited data, but slow (same technology as Verizon, but in practice seems to be a lot slower than Verizon). They also have a significantly smaller coverage footprint than the others. People who are within that footprint, though, and whose budget is more important than performance, will be the ones that would consider Sprint.