Coal is cheap to manage (fuel makes the bulk of the cost) but with massive externalities. Some western countries have become slightly better at offsetting these while providing incentives for cleaner energies, but it isn't the case in many other countries where coal plants have been used to power Bitcoin mining farms.

On the other hand nuclear power has way more fixed costs (infrastructure, skills, waste management) compared to the fuel costs, which makes it less profitable in scenarios where its massive output can't be completely used.