Here's the thing: Currently low level queues are a monster, especially for DPS.
Each low level dungeon is set to run along a very narrow path to 50, where it opens up. The gap between each dungeon bracket is roughly uniform from 15-50. Adding more dungeons, unless they added one new dungeon for EACH bracket (which would be an absurd amount of work) would upset the current balance, and also spread out the number of people queueing for each dungeon.
In short, adding more dungeons would just make queueing for dungeons outside the roulettes more of a pain for lower levels.