Power plants bid for supplying energy at specific time and dates and for specific amount. With renewables you have cheap wind and solar that can outbid everything else in good weather conditions, but in bad conditions you have fossil fuels that will outbid renewables. With nuclear the bids are always the same regardless of weather, and thus if the price is lower than fossil fuels at one date it will most likely be lower at other dates (with exceptions for maintenance windows).

Since natural gas has the currently lowest operational cost of all other fossil fuels on the average, they are usually able to outbid other fossil fuels. However as with any other market, the one that can produce a product cheapest is not always the one that gives the lowest bid and thus different fossil fueled plants get used at different times. Some plants can also produce heat as well as energy and thus artificially lower their bid. Over time however the one who can more cheaply produce a good of the same quality and reliability tend to make more costly production extinct.