I think we would see mining of asteroids by now and the use of that mined material to refuel in space and build space habitats. Nuclear engines are most useful for maneuvering in space, not during launch, since space is already full of radiation so releasing more doesn't really matter. Their downside is the added need for shielding mass on manned missions. Maybe we will see a resurgence in interest towards nuclear propulsion once we get a permanent base on the moon and/or Mars or we are moving asteroids around because then the increased efficiency really starts to matter.