First you need a little history on the Unreal engine to understand why it's so popular. Unreal engine 1 was supposed to be a Quake killer. Their direct (and only) competition was iD software and the popularity of quake, which was the goto engine for developers at the time. Epic was witness to the birth of the modern modding community and as a result were able to build their engine from the ground up with this in mind. They developed a powerful scripting language called Unreal Script just for this purpose, so that a wider audience of amateur developers could mod the game without having to actually mod the game. In fact, the game itself was actually designed around their own scripting language. All of the levels are composed using UScript instead of programming it in, just to make sure their own language was powerful enough to do what they wanted.
As a result, Unreal became very popular and started securing lucrative licensing deals. Epic recognized that graphics engines are far too difficult for most studios to develop, and that a more "user friendly" engine needed to be produced to market this technology around the world. Before this, each company considered a 3d engine their own personal IPO, and didnt want anybody touching it. Your success was tied to the exclusivity of your technology. Epic saw past this and realized that with a powerful enough engine, you could sell it and everyone would benefit. Thus Epic has a longstanding trust in the design world of producing easy to understand, well documented source code.
Crytek still manages to be the de-facto benchmark maker. Nobody really cares that much about what iD or Epic makes anymore, but the world stops when Crytek announces a new engine. It is true that crytek is pushing the boundaries much harder than Epic and has been for quite some time. Rumors are that Crytek's engine is not as friendly as Epic's though, which is an area that Epic has more experience in than Crytek. This is why you see so many more games using UE rather than CE.