New Brevoort Formspring

I suspect it’s a combination of two factors:

1) Most villains are really at their most interesting when placed in context to a particular hero. Sabretooth by himself really isn’t all that interesting, and Sabretooth fighting Iron Man or Spider-Man is just a brawl. But when you put him against Wolverine, their shared history and similarity of powers and backstories really gets a chance to shine. It’s not like you can’t do the former–there have been, for example, perfectly fine Doctor Doom stories that have Doom in conflict with characters across the Marvel Universe. But the best Doom stories pit him against the Fantastic Four–they’re the characters who define him, and vice versa.

2) Once a villain becomes a lead character, you’re forced to spend a lot more time in their heads, and looking at the world from their perspective. And so one of two things tends to happen: either you begin to find more and more sympathetic qualities within your villain, which tend to soften him as a character–those Venom: Lethal Protector stories are a good example of this. Or you don’t, you keep him as nasty and vile and reprehensible as he ever was–at which point a lot of the audience tends to disengage, as it’s difficult to relate to a character like that.

Also, when a villain has an ongoing series, it becomes difficult to pit them against their core hero in a meaningful way again–at that point, both guys have their own titles, so the only real outcome of such a struggle is inevitably going to be a meaningless draw. That too weakens the essential appeal of the villain.