I think the key to the answer to this is in the name of the ship class itself - Nebula Class. I could imagine it was designed with a pathfinder type role to identify and map faster and safer routes through Nebulas in space which a more conventional ship's sensors might not be able to locate. Voyager showed us the value of having an astrometric sensor system, prior to this there were time where they were taking detours and risks trading for maps and other information to find their way through unknown space but the development of an arguably superior sensor system by combining technology they had picked up along the way allowed them to chart shorter courses and safe passage through what would have otherwise been unknown hazardous space. Voyager developed their system out of necessity and opportunity, Starfleet on the other hand - building a dedicated class of ship designed to improve on existing maps would have made sense not to mention both the scientific and military benefits of having a significantly greater mobile senor system at their disposal isn't something to be taken lightly.