Analyst Benedict Evans tries to explain why the iPhone accounts for about half of US smartphone sales, but is outold 2:1 by Android globally. He argues that it all comes down to plan pricing and phone subsidy.

In the US, consumers save US$100-$200 on the cost of the phone, but the plan pricing is the same. As a result, US consumers only save 10 percent by going with a less expensive Android device. Overseas, however, plan pricing is much more flexible. Customers can buy a cheap phone and a less expensive plan to match. As a result, the iPhone can be as much as 160 percent more expensive than its cheaper Android counterpart.

You can read his full analysis in this article on ben-evans.com.