Because people talking about climate change keep sending out contradictory messages. If climate change was as grave they talk about we wouldn't have seen them opposing nuclear and GMO.
They had to shut down some nuclear reactors in 2019 in Norway because the ocean water was too warm. Nuclear requires a highly organized society to function. Nuclear is a huge risk since it's getting more probable our societies will collapse in the future. Who will maintain these nuclear power plants when people struggle to feed themselves? We need to start winding down our consumption, not replace it with nuclear power.