I still fondly remember an online discussion where a new writer asked whether he could give his villain a sidekick, and the first response started with the fervent declaration that of course not, villains are allowed only henchmen, lackeys, and minions in the course of explaining that he shouldn't ask questions like that, he should just write what was right for the story. Which segued into a long thread about what are the differences between such relationships that lead to people using different terminology for them.
After all, heroes and sidekicks tend to be loyal, trust each other, and even regard each other with friendship and affection. None of which are Evil. The Evil group is held together by fear of what happens if they break up, or awareness that their greed can be more easily satiated together (which grates on the pride, and furthermore lasts only until you realize that killing the others would net you a real haul this time), or a common greater hatred, or the like. They would also have no compunctions about harming each other, and no respect for authority.
Of course, those are the classical loopholes by which the heroes triumph: when the villains turn on each other, or desert each other, they leave things wide-open for the attack. Less evil members can be even induced to leave by the boss's attitude toward failure, and claiming of reward during success, or the endangerment of their families, and many others. So it's not given the same sort of treatment as always evil races. At least, not all the time. Many an Legion of Evil, or League of Supervillains, or army of the Evil Empire hits the same failure issues as the Evil Race.