Eric Boyd's comments elsewhere explain why dragons and evil outsiders feature so prominently, but in a nutshell, it's about space. We don't have to reprint full stat blocks from creatures taken directly out of the Monster Manual, and after you hit about 14th level, about half of the available monsters are dragons or evil outsiders. When you subtract the good outsiders (which generally don't make good bad guys), the pool grows even smaller. If we were to, say, go with aberrations or fey or high-level humanoids, we'd have to print a LOT more stat blocks in the last 4–5 adventures, which when you account for the fact that these stat blocks take up SO much room, means that in order for those last adventures to provide enough encounters to take a party up a level, you'd have an adventure that gets into being 30,000 words or more. And since we only have 45,000 or so words to work with for adventures in an issue of Dungeon... well... the problem becomes pretty significant.