Fission reactors were probably deemed too unreliable for deep space probes (too many moving parts for control rods) as retrieving the satellite for repair is not an option, hence the preference for RTGs (zero moving parts).
Uranus and/or Neptune are priority targets for NASA for an orbiter probe, similar to Cassini & Saturn, so watch this space. Any launch trajectory will inevitably require gravity slingshots, probably from Venus and/or Earth.