I think the answer is as simple as it’s been around the longest as an easy to use game engine. Unreal Engine and Torque were around longer but nowhere near as easy to use.
I was first attracted to Unity because it looked solid, UDK never appealed to me because I had the (probably false) impression that it was only good for shooters, and it had all these foreign sounding things like “Cascade”. Maybe others felt the same? I dunno.