It's because the average person has no real knowledge of anything nuclear beyond accidents like Chernobyl and Fukushima, and only get misinformation drip fed to them from the entrenched power providers, who spend a lot of money keeping it that way.
Solar is great, during the day, and when it's clear. The drop offs are pretty big outside that though, and things like battery storage are good but not a full solution, so it cannot be used as a reliable base load source. That's where nuclear can replace the coal and gas plants that produce that current base load, with dramatically less pollution.
No one ever talks about all the shit being thrown into the atmosphere around those burning plants, including radioactive material, because it's not as intuitive to the average person. Obviously nuclear is related to radiation, it's less obvious that burning things like coal does too.