Well, at least for Germany it was the actual nuclear fallout over large areas of the country after Chernobyl. Which is btw still measurable today. [1] That's a pretty scary thing to happen to you and one just has to accept that these are the actual lived experiences of people that form their opinions.
Radiation detectors can detect very low levels of radiation (far below any measurable health effects, for instance), so claiming we can still detect fallout from Chernobyl doesn't really say anything.
> In the last years values of up to several thousand becquerel per kilogram were measured in wild game and certain edible mushrooms. In Germany it is not permitted to market food with more than 600 becquerel caesium-137 per kilogram.
The numbers and actual risks don't matter to change regular people's feelings about a technology. All they know is that there was actual nuclear fallout and now the mushrooms in their forests are radioactive.
I think it was mostly caused by fear about nuclear Armageddon during the cold war - it's hard to feel like the world could end at any second due to nuclear bombs while also feeling grateful for nuclear electricity generation. Would be even if there was no overlap between military and civilian nuclear industries, which of course there is.
[1] https://www.bfs.de/EN/topics/ion/environment/foodstuffs/mush...