The main reason why dps jobs are popular is because there is a low amount of team responsibility. You can jump in and play it like a single player game.

The quality that attracts a specific personality type to tanks or healers is that very same sense of team responsibility. It's the idea that you can make an impact on your teammates to help them succeed. And be thanked for it.

At the moment, the only way that you can help your teammates succeed is by providing a raid dps buff. Anything else (mitigation, enmity, keeping your team alive) is an expectation. Failure to do so will irritate your team, but doing so successfully will never earn you a word of thanks.

If you don't give tanks and healers a way to feel needed and rewarded, the very personality types that would seek out those roles ordinarily will lose interest in playing them. Everyone is focused on where the sub money is, but these are the people you actually need to be winning over. Because when your queues dry up, your new players dry up.