The near-monopoly comes from network effect. There are plenty of other games, I'm more of a painter and have models from a load of them, but I've only ever played GW games as that's what my friends play.

I can't think of anything that comes close on tabletop in terms of number of active players. I've just moved to a new town (pop ~5K), there's a club and that wasn't surprising. I wouldn't expect that with any other wargame.