It's not really a surprise...
1. Apple are American, people in the US are more likely to buy it for that reason, just like how Nokia stayed more popular in the EU for longer than they should have.
2. The Networks in the US are ****ed, you basically have to have a contract over there, when the subsidized prices are rather similar across the board, there is no barrier to entry for an iPhone.
If the 24 month contract went away and people had to buy their phone up front, Apple would be in big trouble and everyone would be buying things like the Motorola X Style or the LG G4 etc.
Over here in the UK I see the LG G4 for £320, the GS6 for £380, the Style X launches at £360, while the apple alternative is £700. However on contract the price isn't really any more for the 6+...
So I dunno how Apple are keeping contract prices down, but I bet the Networks are just jacking up the price of the Androids because they can.