Because the way how the EU electricity market operates first to supply electric power are the power plants with the lowest operating costs. This are usually renewables and nuclear power plants. Both are capital expensive and cheap in operating costs.

Usually the capacity factor of European nuclear reactors is higher than 60%.

olkiluoto-3 nuclear reactor, had capacity factor 70% in the year 2024: https://world-nuclear.org/nuclear-reactor-database/details/o...

Mochovce-3 had capacity factor 74% in the year 2024: https://world-nuclear.org/nuclear-reactor-database/details/M...

In the U.S. they really try to get maximum from nuclear reactors. https://world-nuclear.org/nuclear-reactor-database/details/W...