Please clarify: is it because more people choose to use it as a deliberate act (i.e.: if it didn’t have Android they would have chosen something else that did), or is it because it just happens to be there on more devices (i.e.: Android is irrelevant to their intended use)?
People generally dont buy what they dont want. That is why they aren’t buying Symbian and RIM phones.
There are alternatives available on both major carriers, and it is only correct to assume that the vast majority of the people that choose Android, regardless of the number of models available, did so because they had to like what they saw.