Orion does not scale down very well, the minimum sized start at 10 000 tonnes and much heavier.
Whereas NSWR scales down quite well. The minimum occurs when a critical mass is barely achievable, and that is possible with a mass flow of a few tens of grams per second.
(Actually thrust to weight ratio, but allowed by higher maximum thrust)
Also, the potential maximum thrust from an Orion design is quite limited by vibration and thermal limitations. With Orion, more thrust = more heat in the vehicle. Plus God's jackhammer beating on your skull at a faster rate might overload the dampening systems.
With NSWR, all you need for extra thrust is a faster pump, and a mechanically stronger engine chamber. The heat passed to the vehicle remains constant (although neutron flux may get interesting!). Virtually all of the heat in a NSWR exits with the exhaust, thus not bothering the vehicle itself.
silly real reason
If you have a whole civilization employing Orion and Nuclear Salt Water Rocket all over the place, then obviously your species is not Human. Or even any form of carbon-water-protein life. Rather they would have to be some form of radiation-eating BEM.
The environmentalist lobby could be demanding the return of NSWR simply because these newfangled super-efficient Orion drives are simply not seeding enough nutrients into the spacelanes. Why, last quarter's neutron count is below 4e10 per cubic meter already, and there's virtually no sightings of free antimatter being reported at all! We must drastically increase the nutrient release before travelers start starving on the flight to Blito-P3. Never should have scrapped those rest-stops with their fast breeder drive-throughs, I tell you!