what made Starfleet decide Ben Sisko was the best choice for the difficult mission to Bajor?
We don't really know Starfleet made such a decision. It might be that there was no particular incentive to choose the best; rather, somebody who wasn't useful in any other task (as Sisko's therapist after Wolf 359 would unprofessionally but practically squeal to the Commander's superiors) could be dumped there.
The assignment to manage the station for Bajor may have been intended to be relatively short. Get in, wait for Bajor to regain its industrial base and self-confidence, have them kick Starfleet out and shout curses after the departing runabouts, and begone. No hope of Bajoran membership, ever, considering how hostile the locals were - and good riddance to the worthless planet anyway.
Without the appearance of the wormhole and the involvement of the Prophets, Sisko's assignment would probably have ended in something like the Circle Trilogy anyway, barely a year into the mission. Cardassians would probably have been secretly or openly manipulating things even without the added incentive of the wormhole, and Bajor would soon have fallen into their sphere of influence again. And Starfleet could have calculated that sending good and competent people there would be a waste of material.