An internal Apple study from 2011 reveals that the number one reason people ended up buying an Android phone after considering an iPhone was that they didn’t want to leave their wireless carrier.
The chart below is based on data that came out during the Samsung-Apple patent trial (via CNET). It suggests that if Apple had been on all carriers in the U.S. Android would not have become as popular as it did.
Alas, Apple was not on Verizon, Sprint, or T-Mobile. This gave Google an opportunity to win over new customers. Those customers are likely coming off their first Android contracts now. It will be interesting to see if they stick with Google, or if they decide to jump to the iPhone.