Wow genuinely surprised that AWS is not using CodeCommit
If you want open source contributions, AWS' Code* offerings are a terrible choice. For example, AFAIK you still can't display a build status page that people can view without being logged into the AWS console, like Travis CI or AppVeyor. I honestly don't know who they built those services for, but I'll take Travis CI or Jenkins over those services any day.
People who aren't developing open source software and people who want integrated IAM, generally. Git repos having ARNs is pretty neat
Similar experience for other build tools tied to IAM. It's not that IAM is bad or anything, but there's always some convoluted way to get git or docker or kubernetes working when it's tied to AWS.