This is because more people can do AC than Arah, and Arah is often considered the hardest explo dungeon (thank you giganticus lupicus), making it not very pug friendly...
And there is a DR system on dungeons if you run a dungeon to many times DR kicks in and you start getting shitty rewards, granted this is also for the same path in the dungeon and you need to complete it under X time, so the average person likely wouldn't hit it.