I've never actually looked into it, so i may be wrong, but Coordinators aren't usually Asst HCs, isn't one of the position coaches usually Asst Head coach? not sure of the reason for that...
Probably b/c they coordinator already has enough on their plate and they aren't able to take on the additional responsibilities as it would take away from their gameplanning. A position coach would have more available bandwidth to assume additional duties. A ST Coordinator as Assistant HC makes sense b/c ST Coordinators already address the entire team so adding Asst HC if their are capable makes sense.