Renewables (and nuclear) have high up front costs and comparitively low marginal cost. Comparing it against something that burns expensive fuel can be misleading (if you want it to be).

One system is taking the money and turning round and handing it to their fuel suppliers, the other is turning round and handing it to the people who loaned them money to build a giant complex structure that would take a decade or more of risk and uncertainty to pay itself back.

Gas bids high because it literally makes no business sense for them to operate for less than the gas input costs. They'd rather sit it out when electricity prices are lower. Renewables will take almost any price once built because wind and sun is free, but won't build in the first place without a path to paying off their investment over a certain timespan.

(This is why more solar has been built in places with good low risk finance than where the sunlight resource is strongest)

CfDs are a competitive bidding process and so give a market price for what developers need their price to be over 15 years or more for it to make a standard return on investment. Developers outside this process need to guess what electricity prices will be, at the times they generate for years to come.