Historically I think people have just liked an enemy which they can go and fight without moral questions. Like with human bandits you will get half way through the fight and they will surrender and say they have families. Which instantly turns it into a moral dilemma. Of course with orcs they are now depicted as clearly having families, and just like a human bandit, an orc, goblin, or kobald bandit can pull the exact same card.
5e gnolls and mind flayers on the other hand still allow the 'brains off combat/dungeon crawl' kind of approach. Though mind flayers themselves introduce body horror which in itself can make it unsuitable for some players. They're also quite psionic themed, so some DM's might not want them in their setting. And though 5e gnolls are depicted as literally demons who spawned from dead bodies eaten by hyenas, the fact that other editions and other media has shown them as a more standard species makes me suspect that they are on their way to being PC's too.