IMO - Starfleet originally looked at DS9 as a backwater posting and a symbolic gesture to Bajor as opposed to a more meaningful operation. When the wormhole was discovered Starfleet assumed that Sisko would be more of an airport manager and that exploration and first contact would be handled by ships transiting thru. It was also , of course, an olive branch to Bajor when Sisko was proclaimed the Emissary.
As Sisko began to prove himself it became more difficult to replace him both due his own actions as well as the complexity of his role in stabilizing Bajor after the occupation. To pull Sisko out would just make the recovery of Bajor that much harder.
I suspect that if the Dominion war had come sooner that Sisko would have been replaced but by the time hostilities broke out he was already a proven asset.
As Sisko began to prove himself it became more difficult to replace him both due his own actions as well as the complexity of his role in stabilizing Bajor after the occupation. To pull Sisko out would just make the recovery of Bajor that much harder.
I suspect that if the Dominion war had come sooner that Sisko would have been replaced but by the time hostilities broke out he was already a proven asset.