They are trying to move from their C-based implementation to Chez Scheme for maintainability. Since Racket programmers are familiar with Scheme, using a Scheme for the implementation has obvious advantages. They're also using it as an opportunity to "do things the right way this time".

Along these lines:

> Putting a number on maintainability is less easy than measuring benchmark performance. Anecdotally, as the person who has worked on both systems, I can report that it’s no contest. The current Racket implementation is fundamentally put together in the wrong way (except for the macro expander), while Racket CS is fundamentally put together in the right way. Time and again, correcting a Racket CS bug or adding a feature has turned out to be easier than expected.

Last year's post has some details on the organization of the Racket codebase and why some of the layers are being moved to Chez.

https://blog.racket-lang.org/2018/01/racket-on-chez-status.h...