Maybe that's the thing -- the world is *so* large and diverse and *so* malleable that it lends itself to crossovers too easily. So, writers who wouldn't ordinarily grab a crossover 'verse that was difficult find that HL is easy to incorporate, and hence the popularity!
I mean, it's an easy gimmick, right? If your favorite character gets killed off in Universe A, then write and AU crossed w/ Highlander and bring them back as an Immortal, and presto-chango, new life!