Since lvl 25 onwards, you will see people calling for tank and healer in dungeon. It is understandable that they don't queue themselves anymore.
If the queuing system can allow you to see how many ppl on queue as well as if they are DPS, healer or tank. There may be more queuing of dungeon.
I am suggesting this because i can see some ppl in this game are not very vocal and they are passive. they might miss out dungeon runs.