The uniqueness of Data was almost certainly a factor, and I’d also argue that Commodore Oh only (presumably) reaching her place in Starfleet after his death played a factor, that inserting an operative in Starfleet Intelligence and having them rise through the ranks takes time.
With the Romulans having been in isolation for fifty-ish years prior to TNG’s first season, Data’s discovery and joining Starfleet might have been off their priority list, and while inserting an operative into Starfleet, getting them to a position of being able to order him into the waiting disassembly team of Zhat Vash agents would then gain focus, it would take time, time in which Data ended up sacrificing himself on the Scimitar before it could come to fruition.