I'm not sure how the other actors fanbases works, but JM has a very loyal following, who are usually fill up every gig and convention he does. Many travel abroad, sometimes even across the pond, to see him. So it ciuld be that many bought tjis statue, not owning anyting else sideshow, because a)they're big fans who don't mind spending money on JM stuff and b)this is the most lifelike and character accurate statue thatbwe are ever likely to see. Not that you need to be a Spike fan to own this statue, just a general JM fan, since the vamp head is optional.
Hopefully, this will be true for a lot of people, and once they have Spike on their shelf, they'll think he's lonely and buy the rest of the line, even though they didn't use to be sideshow fans.