When I used to queue for this as a DPS, I would become #1 in the queue but still had to wait over an hour. The average time, which kept updating, indicated to me that parties were successfully forming for it, but that I was not in them.

After I got in and completed it, I looked at my contacts list. Each and every tank and healer shared a world with a respective DPS. Only one of the tanks or healers queued solo and that's why I was able to get in.

A lot more people play DPS so it's with them all trying to do the content, there may not be enough tanks or healers to match with all the DPS, but the issue I experienced is a common reason as well.