It's a carryover from when 50 was max level. Since you had to be 50 to enter those dungeons, and you couldn't level past 50, there was no need for the mobs to give exp. Hopefully the address this soon, because now that we can level past 50, it's pretty silly to not get experience in these dungeons.

However, do note that you can still get exp from the new post-50 dungeons.