It was meant to be a backwater station around a planet that was unstable. Mostly they job was be in the way of cardasian return and try valiently to get things towards Fed membership. I believe he had been a Lt commander aboard ships serving as the XO. He was at least a project lead at the ship yards. He may have even run the yards, I don’t recall right now.

He was pushed into a command role that was intended to bring him back to where his career was many to be.

The rank of a station commander depends on the importance of the station in the fleet. Admiralty are posted to stations that are region command stations. The postings serve as command of the fleets in the region as well. For smaller posts the rank to command them drops a lot. Sisko was placed in command of one small station with a couple runabouts. The rank of Commander suits the post. Once it got more important he was already imbedded deeply with the the bajorans.