Tanks get instant queues for nearly everything, so simply put, there aren't enough of them, especially compared to the ratio of DPS queuing. This was something implemented to get more people to try tanking. Whether it's had the desired results, it's hard to say.

You still get tanks dropping queues if they don't get something they like, but this is a player created problem and not something SE can fix, apart from placing harsher penalties on ditching.

You can tell the people who picked up a tank class purely for convenience or necessity over those who genuinely like it.