Another factor is the fact that they are working at night. And Europe is further to the north than California, nights has a tendency to get much longer in winter (up to 16 hours of night in Central Europe), so cheap energy from solar panels is instantly nullified by lack of sunshine during winter, when the energy is needed the most.
Poland gets on average 1 hour of actual sunshine in December. I'll take nuclear power unless somebody really has reasonable strategy for getting 3-7 days of energy storage.
Hydro and gas both involve storage (of water and methane), and both can be, and are, used with renewables and/or nuclear to fill gaps between supply and demand.