Good question, I wrote a more detailed answer to that: https://erik-engheim.medium.com/why-is-the-neutron-rocket-using-carbon-fiber-composite-instead-of-stainless-steel-fd9f3bb8b24

I am not certain about this, but I am not sure if SpaceX used Automatic Carbon Fiber placment which reduces costs a lot.

The material itself is not that expensive. It is typically the process which makes it expensive. The need for molds also ads cost. If you don't change the molds often that is not a major concern. However SpaceX likes to do a lot of hands-on experimentation. Carbon fiber is not well suited for experimentation. You cannot bend, cut and weld it like you can with steel.