I always wondered this too. Healing has a higher population but I thought that just because there are more slots offered for them in content like 24 man where there are 6 healers and 3 tanks.

But tanking IMO is much easier than healing. Healing is stressful IMO. I think it is just intimidating to people.

People want to follow the leader while people who tank like me want to be the first person to hit the enemy.

But really both roles when they make major mistakes are noticeable and I think when people see that or experience that mistake it can persuade them to not continue or not choose to play that role.