Tanking is hard, or it's perceived as hard. Some people enjoy the extra challenge, most shun it.

Ideally, a party should be able to launch without waiting for someone take the "terrible" role of a tank.

By making tanking easier, hopefully more people will do it. I am not a fan of the holy-trinity anymore (tank, dps, healer) but WoW is built on it and this is a good solution for the long DPS queues.