For the vast majority of heroes, many if not all of their enemies will share similar powers, backgrounds, and personalities as the hero. Speedsters will face other speedsters, psychics will fight psychics, robots will battle robots, and badass normals will fight other badass normals. Deeper than that, villains will often have similar motivations and personalities to the hero as well. A light-hearted, jokey hero will get a lot of equally light-hearted villains with which to exchange insults, while a dark, broody hero will get a Rogues Gallery full of borderline personalities with which to have dark, nihilistic discussions mid-battle. A hero with an animal theme will end up being constantly annoyed by animal themed villains, while an elemental themed hero will always find bad-guys with comparable elemental themes.

Even when characters are known to live in a shared continuity, such as the DCU or the Marvel Universe, villain types will rarely leak from one comic to another -- Spider-Man rarely finds himself up against the Powered Armor-wearing villains that Iron Man faces on a daily basis. Of course, a shared continuity makes this much easier to justify, too. Perhaps the reason Spider-Man isn't going up against the Powered Armor villains is because Iron Man already has them handled.

There are a number of editorial reasons for this phenomena:

  • If a reader enjoys giant robots or motorcycle-riding badasses, he's probably going to be interested in seeing more of the same.
  • It might be easier to work with suspension of disbelief if the character doesn't have a hundred different sub-genres wedging their own mythology into his story.
  • It can be difficult to write plausible stories for a character with more gimmicky powers. A flying brick is effective against a range of adversaries, but if your only power is psychically controlling cupcakes, you will more likely than not find yourself facing a wide range of fearsome baking-related foes.
  • If the hero is not all that powerful, putting him up against a galaxy-eating cosmic horror is going to end in tears -- Galactus would will Captain America in a fight. Mismatches like that are just asking for trouble.
  • The reverse is also true. A powerful hero taking on villains way below their weight class does not play well into drama. Superman vs. purse snatcheres is sort of humiliating for everyone involved.

Additionally, there are several in-story justifications:

  • There is only one basic type of enemy, or only one type of superpower exists.
  • The hero's job is to fight this one type of enemy. Other enemies fall under other jurisdictions.
  • The hero may be obligated to fight one type of enemy for some reason.
  • The hero's powers may only work on a certain type of foe, or are somehow limited.

Of course, pitting heroes against villains that generally outpower them can be a good way of spicing up an ongoing series. True moments of awesome can result when this is handled well and the hero comes up with a creative way to beat the bad guy.

In some series, particularly long-runners that have been developed over the course of years (or even decades), some members of the Rogues Gallery may not fit the theme. Batman, for instance, has gathered a respectable number of enemies who have actual super powers, even though the overall theme for his Rogues Gallery is that of the crazy badass normals whose crimes are based on some specific theme.

Finally, depending on the hero, his or her [{Rogues Gallery]] may have multiple themes. Not all of Spider-Man's enemies fit into the Animal Motif theme. The ones that don't tend to be the results of scientific experiments gone horribly wrong. Indeed, some Spider-Man villains (such as Doctor Octopus, The Lizard, and The Scorpion) fit both themes.