One real-world reason that the world isn't anyone's oyster. Except ours of course and there is some uncertainty even on that point.
1. Catastrophe - With the best will in the world, if Nature says you're going down in the fifth, then that's it. Earthquakes, volcanoes, tsunamis, hurricanes, even meteor strikes are hard to parry with your changing shape, mental attacks or fiery breath.
Now let's take two classic enemy within species.
Dopplegangers - five reasons they haven't taken over yet.
1. They have to revert sometime. You can guarantee when this happens someone has cast a ritual or is using an item to scry on them. Big secret out, time to leave town. Robert deNiro's character in Heat says it best.
"Don't let yourself get attached to anything you are not willing to walk out on in 30 seconds flat if you feel the heat around the corner."2. Undead. You'd be amazed how being able to impersonate someone flawlessly makes no difference to a ghoul. You'll still taste like chicken to them - it might be useful if you've got a friend able to rebuke them into leaving you alone? No? Oh dear.
3. Rakshasa. They play the same game, just for higher stakes. Nobody likes competition and in a fight, my money is on the tiger-men with the funny hands. It'd be a rare doppleganger to beat a rakshasa wouldn't it? Maybe with a lot of advantages...
4. Dragons. You really think parlour games would impress them? Slow exhalation...
5. They make phenomenal actors. Why take over the world when they have the hearts and minds of their audience, invitations to the best parties, romances with beautiful admirers? And where better to hide their collection of costumes? Never forget a line either...
Mind Flayers - five reasons we're not all brain food.
1. Githyanki. They have psychic mojo, big shiny swords and pact dragons. The last is a kicker.
2. Githzerai. Psychic mojo. Kung fu. Hardcore magic. And that first hit is going to hurt.
3. Golems and Guardians. Not another fantasy heartbreaker but the monsters. Spellcasters can make things that are a real pain in the cranium and which will floor most minions. This means that they of course must be the first thing to go come the revolution.
4. Aboleth. They play the same game and their adaptation to water makes them deadly in that environment; their slaves appear nutritious but contain their own hazards. The slime mages and overseers can usually out-psychic a mind flayer of any stripe outside the Forgotten Realms.
5. Treants. Hoom. The idea of a vegetarian illithid (treants and dryads only) amuses me but the animated trees are going to make it a bad day for our tentacled friends. A mind flayer may find the trees a hard target to mind blast.