Simple answer. People from the northwest love the northwest and want to be based there. Horizon is a great company who treats their employees well especially in comparison to many of the airlines now days.
It's not always a race to the first legacy airline who will take you for everybody. If you're looking for a comfortable living while you're in the area you love then it's worth a shot.
With that said I fly cargo for a UPS feeder so I guess I don't have much experience for input. But I do love my home and would love to work at QX for those reasons.