We currently have 24 Base Game Group Dungeons with 2 Daily Pledges that repeat every 12 days, and 30 DLC Group Dungeons with only 1 Daily Pledge that repeats every 30 days.
For example: Today's Daily Undaunted Pledge Quests are Fungal Grotto II (Base Game), Crypt of Hearts II (Base Game), and Falkreath Hold (DLC). We will see the Daily Pledges for Fungal Grotto II and Crypt of Hearts II again in 12 days, but we won't see Falkreath Hold again for another 30 days.
This is too long of a stretch for the DLC Pledges to repeat, and that stretch just grows with every DLC Group Dungeon that gets added to the game...
If there was 2 Daily Undaunted DLC Pledge Quests, then it would only take 15 days for the DLC Pledges to repeat, which is closer in line to the Base Game Pledge calendar.
PS:
I know we can queue for any Group Dungeon at any time, but the point is that it takes too long for the DLC Dungeons to cycle as Daily Pledges. Whenever a Group Dungeon has a Daily Pledge it's usually faster to get into it using the Activity Finder because more people are queuing for it. I've even queued for Specific Non-Pledge DLC Dungeons on a Tank and have had to wait over 20 minutes for the queue to finally pop, but sometimes it never pops at all! So if the DLC Dungeons cycled as Daily Pledges more often then everyone would be able to get into those Dungeons more often.