The simple fact is that Intel has put a heavy focus on their higher-end server processors, and it shows in their market share. Through some creative marketing and performance-oriented updates to their lineup, they've become the trusted go-to brand for many providers in many industries - be it dedicated, vps, web hosting, or cloud. At this point, it's "If it ain't broke, don't fix it.".
I personally feel that Intel processors offer a better overall value and package, but I'll also say that I've never seeked out or really even used any AMD Opteron-powered servers to compare them in similar configs.