It might be just a decision to own the code as it probably ends up in production, e.g. run codeql and other tools to scan it, have controlled releases and limit

It might be just a decision to own the code as it probably ends up in production, e.g. run codeql and other tools to scan it, have controlled releases and limit access to the repo. They might have had some other stuff to change and did not want to bother doing it in the original repo with unexpected timelines from the repo owner. A fork is a logical step for a company.