Could it be because nuclear is highly centralized? I would expect that something like solar/wind power would be better for decentralization (in a war).
Even if you don't blow up a nuclear plant, it seems like cutting the power from one would be relatively easy.
Russia has refrained from hitting Ukraine's nuclear plants directly, and Ukraine has more or less kept them connected to the grid (albeit with nonstop repair efforts).
Transformer substations are more vulnerable targets but it's hard to be decentralized enough to not have those.
Even if you don't blow up a nuclear plant, it seems like cutting the power from one would be relatively easy.