Why?

Because not everyone is good at all jobs in a group. Someone might run as a tank and always get 3/4ths (as usually happens) of the commendations of every run then switch to a healer and suck at it. Then we're stuck with shitty healers that get high priority queue, because while they might be a good tank or an ok one, they just don't have to chops to heal or dps. Take me for instance, in most games I'm a really good tank and a decent healer, but I suck at dps usually. Is it right that I get that priority then because I tanked or healed for the first few months of playing?

It also gives people the ability to rack up commendations then once they're high que priority start acting like little jerks.