Bitcoin is an ecash protocol involving a currency and a network that are both called bitcoin. That protocol deserves a dedicated github page.

Bitcoin Core is one implementation of that protocol in the C++ programming language, and that deserves a separate github page.

Although that is a nice theoretical way of distinguishing the projects, it is worth pointing out that due to historical reasons, the "bitcoin" page on github hosts the actual C++ implementation of Bitcoin Core (partly because it was written first, before any specifications existed), and the "bitcoin core" page does not host that. Instead, the "bitcoin core" page hosts some related parts of bitcoin's tech stack, including a debugging tool and bitcoin core's cryptography library.

Maybe someday the github pages will be refactored so that the bitcoin page only hosts the BIPs and the protocol specification and the C++ code will go onto the bitcoin core page. But I don't think that will happen because there's not really much point and it would generate a lot of short term confusion.