Because it doesn’t help. Renewables want to be paired with something that can easily be spun up and down as needed. Nuclear doesn’t fit that model. It tends to make it worse, because cheap energy we could be getting from solar or wind has to give way to the nuclear baseload instead.
It’s something of the opposite problem of the sun not shining at the same time the wind doesn’t blow. At times where you have tons of both, you want to store them up for later. Nuclear forces a situation where you have to do that even more.