Roku uses Gitlab internally so could just be a standardization thing.

Moving changes back and forth between gitlab and github is a pain. Could indeed be as simple as standardization and reducing development pains.

What makes it more complicated than doing a Git push against two remotes? Assuming that one of those is used for collaboration and the other is just a mirror.

The CICD pipeline is not directly compatible, for one.

You don’t need to run the CI/CD pipeline in both. Choose one as your collaboration space, and the other as a mirror.