Because solar and wind are renewable, cheaper and cleaner than nuclear. They don't require destructive mining for enriched uranium or create the security implications of dealing with fissile material. Solar/wind do not create long term hazardous waste that's complicated to dispose or create the risk of widespread radioactive fallout. They also help to decentralise the energy grid making it less dependent on a single point of failure.

Nuclear power has its advantages, and may be worth it short term because climate change is a threat to humanity, but nuclear is not a renewable resource. Solar/wind with proper recycling could in theory sustain itself into perpetuity. Humanity needs to find sustainable ways for powering itself in the long term.