I can't imagine some people being able to clear a dungeon if it was tuned up to the difficulty of a HM Primal. HM Primals just aren't for everyone. Dungeons are meant to be able to jump in, learn boss mechanics on the fly and come out victorious. HM Primals just aren't like that, and neither are EX Primals, or Coil of Bahamut. Trying to make the early dungeons harder just to make it a smoother transition into the harder content just wouldn't be right in my opinion.

There's Crystal tower, Srycus Tower, myth and soldiery gear. All of which you can get without too much dying to boss mechanics.