The biggest factor here is that this is the first U.S. iPhone launch on multiple carriers. That's more than double the addressable market, and I'd imagine that

The biggest factor here is that this is the first U.S. iPhone launch on multiple carriers. That's more than double the addressable market, and I'd imagine that the effect on purchasing decisions goes well beyond that. In short, forcing users on one carrier made the iPhone something of a niche product, but now it's mainstream.