Because it was designed with the capability to solo it, and to work with any composition with a change of tactics. So the DF is setup to allow that as well. Otherwise it would be too easy if you had a standard setup.
Almost every time you get a standard party setup the dungeon goes by really fast tanks pull big, DPS nukes them, boss fights go by really easily due to everyone falling into standard dungeon routine.
A nonstandard setup causes the party to adapt on the fly to their durability and kill speed.