Cost is a factor but not the only factor. Intermittency is a factor. Geographic limitation is a factor. Land consumption is a factor.
Cheap intermittent energy is an okay supplement, but cannot reliably deliver when it is needed. Hydroelectric and geothermal are great, non-intermittent clean energy but are impossible to build without the right geography. Fossil fuels are cheap, and deliver power anywhere but emit carbon. Nuclear power isn't as cheap as fossil fuels or intermittent sources. But it's the cheapest non-intermittent source that isn't geographically dependent.
If our goal is to fully replace fossil fuels, then nuclear is the best option (besides building geothermal and hydro where we can). Sure, solar thermal can deliver clean energy without the need for additional energy storage. But we could build twice as much capacity with nuclear and use a fraction of the land, and avoid having to build larger plants in the north and south, and avoid seasonal output fluctuations.