The short answer is reliability per watt. Solar's got lots of ups and downs which means designing bigger than you need. Batteries need replacing because they don't last forever. Solar panels have to be kept clean of dust on planets like Mars. Solar panels have to be deployed, which requires additional mechanisms that may fail or require human intervention, and so on.
The way they want to design reactors for next generation space exploration, they're set-it-and-forget-it designs. They have extremely few moving parts, and basically as soon as they're uninhibited, they'll run until something fails and their service life ends - likely several decades after construction. They're extremely reliable and are basically black-box sources of electrical power. Once they put it on the moon or Mars, they can just bury it under the regolith and run the cables back to where the habs are and forget about it. When it eventually fails, they can just leave it in the ground.
The biggest draw for nuclear power is continuous, autonomous processes, like thawing Mars permafrost for water, electrolyzing some of it for breathable oxygen and hydrogen, and using the hydrogen and carbon dioxide to make methane for return mission fuel for the complete in-situ resource utilization mission profile. These processes would run as soon as they were setup on Mars, without human intervention, and could run for months or years before humans even arrive, requiring no human intervention. Doing the same thing with solar would mean contesting with intermittency and having to actually assemble and maintain an enormous solar farm on Mars autonomously... We can't even manage that on Earth.
The way they want to design reactors for next generation space exploration, they're set-it-and-forget-it designs. They have extremely few moving parts, and basically as soon as they're uninhibited, they'll run until something fails and their service life ends - likely several decades after construction. They're extremely reliable and are basically black-box sources of electrical power. Once they put it on the moon or Mars, they can just bury it under the regolith and run the cables back to where the habs are and forget about it. When it eventually fails, they can just leave it in the ground.
The biggest draw for nuclear power is continuous, autonomous processes, like thawing Mars permafrost for water, electrolyzing some of it for breathable oxygen and hydrogen, and using the hydrogen and carbon dioxide to make methane for return mission fuel for the complete in-situ resource utilization mission profile. These processes would run as soon as they were setup on Mars, without human intervention, and could run for months or years before humans even arrive, requiring no human intervention. Doing the same thing with solar would mean contesting with intermittency and having to actually assemble and maintain an enormous solar farm on Mars autonomously... We can't even manage that on Earth.