It's mostly because it's defined by pickup position rather than brand, so for example a P bass sound can be a Fender or an Ibanez, doesn't really mater it's the position that gives that ballpark character. Smith have (nearly) always used such unique and specific pickup placement that it has a completely different character which really sets it apart. That's not to say some basses can't do a 'Smith Sound' but I guess there are only so many ways you can place pickups and Smith did that first, or most successfully.