Maybe a better incentive would help, mean mounts are a bit meh, not like you can show them off in instances lol. Maybe they should of thought of adding unique armor for tanks for tanking certain events. Like if you tank coil 1-5 on warrior you get x vanity body piece and if you do it on paladin you get y vanity body piece.
If you tank extreme primals on warrior you get x ifrit armor and if you do it on PLD you get y ifrit armor. I would say an incentive like that would entice people to queue tank more and then they could do the same thing for healers, give them something they can show off for vanity, the mounts just don't really have a huge sense of "I need it" as much as a unique vanity piece would for some.