American consumers are a bit less fickle than Europeans. That's why Apple doesn't grow large marketshare, but they never lose any, and Android is gaining the way it is. People here are generally not likely to consider alternatives as much, especially when almost all those phones come with locked 3G bands or from a carrier on contract.
If they just bought phonex, they won't care that Microsoft has great advertising. The amount of the market that chases technology the way many early adopters here do, is very very small.
I don't think we'll see the full impact Nokia and the better WP7 devices (and later updates) have on the market until a lot of those contracts fall off next year...