Not sure. I could imagine it being because it is no longer a Scheme, or the creators and maintainers do no longer want it to be seen as a Scheme. Or perhaps because it introduces differences not adhering to the Scheme standards and therefore Schemers do not like it.

I liked Racket. I only found it difficult to use multiple cores dynamically with it and its places concept and lifting and multiple Racket VMs and all that. Guile seems easier in that regard. But Racket has typed racket and more advanced macro system ...