Except that would be "noooculer", and we can't have that.
Despite the protesters, it's not a huge issue.
New Horizons, Cassini, and Galileo and other RTG equipped probes were all launched despite the opposition. It's just the overall political will to make such a mission. When a mission that calls for a nuke gets funded, we'll have it. I'm not overly concerned about environmental activism blocking it.
The real reason is that the "little portable nuclear reactor" industry is just not one that talks about itself all that much.
Mainly because aside from outer Solar System missions the main consumers of RTG's and small self-contained reactors are nuclear submarines and spysats that don't want solar panels reflecting/giving away positions.