Intended audience, how it works and what suite of products they belong to.

Cloud Storage is part of Google Cloud and designed to integrate with other parts of the cloud infrastructure, and I believe Firebase as well. The audience is developers and people who are comfortable using command-line tools or creating their own applications to interface with the different storage buckets in their accounts

Google Drive is a consumer product designed to integrate with GSuite, the audience is consumers who may be technically proficient. There are tools to access your content and the documents in your Drive folders are usually easier to share.