I work in the industry and got my start with a cooperative. There’s some federal incentives/benefits for co-ops baked into obscure tax exemptions, but really it just comes down to not gouging customers and cautious long term investments in equipment and people.
I don’t have any firsthand experience with Plumas-Sierra but from their rate sheets they cost about the national average despite not having super profitable load centers like San Francisco or owning ~1/3 of the highly profitable transmission that brings power from Oregon to SoCal. I couldn’t imagine what good PG&E could have been doing with all their revenue instead of paying dividends and stock buybacks.