It's mathematics. Each group needs 1 heal, 1 tank and 2 DPS. SO, each tank and each healer queueing can only "host" 2 DPS. which means that in a perfect world, there'd be the double the ammount of players playing as DPS than there is of tank/healers.

HOWEVER, in FFXIV we have
- 2 tanks (PLD and WAR)
- 2 healers (WHM and SCH)
- and *6* DPS (NIN, MNK, DRG, BRD, SMN and BLM)

So, yeah, there are much much much more than 2 DPS for each tank or heal. That being, all those DPS need to wait till there's a tank or a heal available for you, quite possibly on a first-come-first-serve basis. So, there's nothing much to be done to help that, short of stimulating people to play more as tank/heal.

maybe Heavensward and the DRK will help with that. Since it's gonna be all the craze