As far as I know, DPS queue times haven't changed much, if at all, from 2.0 so I don't think there's an issue with tanks preferring one or the other; it's just the general lack of tanks. It could also be an isolated case where a group of people are just leveling tank without actually wanting to play them - I know I'll be doing that when I get around to leveling GLD/MRD. Not saying that you're right or wrong, just that these are other possibilities.