Sisko was appointed as the commander of DS9 because he was highly regarded by Captain/Admiral Leyton
As the previous answer states, Benjamin Sisko was given the command of Deep Space 9 while having the rank of Commander.
At the beginning of his career, he was more interested in engineering than in command. It was then-Captain Leyton of USS Okinawa that promoted him to the rank of Lt. Commander and position of Executive Officer.
LEYTON: Ironic, isn't it? When you came on board the Okinawa, you were more interested in engineering and ship design than command. But I promoted you to lieutenant commander, gave you the post of executive officer, and taught you everything I knew about being a leader.
DS9: Paradise Lost
Which would mean that Capt. Leyton was satisfied with Sisko in the command division and saw potential in him.
Later on, Benjamin Sisko was given the position of XO on USS Saratoga. Which would once again suggest, that he was still a good material for a Captain, or the position of commander of a station/ship.
After the events of Battle of Wolf 359, Benjamin Sisko was given an assignment at Utopia Planitia shipyards.
PICARD: [...] I have been made aware by
Starfleet of your objections to this assignment. I would have thought
that after three years spent at the Utopia Planitia yards, that you
would be ready for a change.
We can see that at least Capt. Picard thought that a change in position would be suitable for Sisko, even if it was only his personal opinion (it might not be included in the decision process).
LEYTON: Captain Sisko. This is my adjutant, Commander Benteen. Benjamin was my executive officer aboard the Okinawa. And a damn fine one too.
SISKO: I did all right.
BENTEEN: Ah, don't be modest. Admiral Leyton has had his share of executive officers and you're the only one he ever speaks fondly of.
LEYTON: Present company excluded.
SISKO: Admiral Leyton is the one who recommended me for the job on Deep Space Nine.
LEYTON: One of my better ideas. You must be Odo.
I think we can reach the conclusion that since Sisko and Leyton served together on USS Okinawa, until DS9: Paradise Lost (or maybe even further in a way), Benjamin Sisko was highly regarded as an officer and commander of Deep Space 9 by Captain/Admiral Leyton and it is stated clearly that his recommendation was a major factor in choosing the commander of the station.