I think it was both.

Fossil fuel companies opposing it I can understand: it’s a straightforward, strategic move.

For green proponents, I think it was an own goal, likely driven by a mix of fear and failing to realize that people would not readily temper their demand for energy and the holistic solution needs to include humanity harnessing substantial amounts of energy. Ruling out nuclear directly (in a single step) means decades of people creating gigatons more emissions from burning fossil fuels.

Back when global warming wasn't really widely discussed, or even believed, nuclear power was seen as quite threatening to a environmentalist.

After all, nuclear power was brand new. And the pollution and death if one goes bad...

Also anti-nuclear greens today and then were looking at significantly different tech. I mean, some of that started when the core catchers weren't a standard thing.