Being free is a big reason, in the same way that cheap OEM licenses are Windows' way of having the biggest market share. Free's not the killer, being cheap enough to be widely available is. Even if Macs or iPhones are the better product, Windows and Android both have nearly every hardware maker in the market selling it (and, in Android's case, on every network), making it a more convenient and marketable option.

AT&T being a crappy network is analogous to AutoCAD and games only being available on Windows; a way to leverage wide availability into concrete advantages for some segments, despite having a worse product to begin with.