We've generally heard that developers move from Bitbucket to Cloud Source Repositories because they want to use other Google Cloud services and it's simpler for them to manage their source in the same place where they debug, build and deploy that code. Developers often mention that they appreciate the unified identity, IAM permissions, and integrations that Cloud Source Repositories has with Cloud Shell, Cloud Build, Cloud Debugger and others Google Cloud services.

However, developers don't need to move from Bitbucket to take advantage of the utility provided by Cloud Source Repositories. You can mirror any number of your repositories into Cloud Source Repositories and take advantage of the code browser, code search, and various integrations with Google Cloud Platform.

(Disclaimer: I work at Google and am the PM on this product)