> Although there are many different implementations, the original implementation, Bitcoin Core, is by far the most popular, and it is used as the reference implementation

https://river.com/learn/what-is-bitcoin-core/

Since it is the reference implementation, other clients have to follow its lead. And that's the lead of a fairly small group:

> Over the years, the code has had more than 800 contributors. However, only seven people serve the role of “maintainers” nowadays.

https://bitcoinist.com/who-funds-bitcoin-core-developers-her...