Oh I don't know, it's not completely obscure. Tools like DrRacket need a community to exist, they don't come from nowhere. It's used at a reasonable number of schools, and even has a book about interpreters written with it (by Shriram Krishnamurthi).
Now if you're asking why it hasn't replaces Scheme, I personally think it's (1) because Racket is yet-another-lisp-derivative, (2) because adding objects to lisp is like a rite of passage for lisp programmers, which trivializes many (cool) features in racket, and (3) because it's hard to convince old-school lispers of anything, let alone that their dialect is wrong, or needs improvement.
Now if you're asking why it hasn't replaces Scheme, I personally think it's (1) because Racket is yet-another-lisp-derivative, (2) because adding objects to lisp is like a rite of passage for lisp programmers, which trivializes many (cool) features in racket, and (3) because it's hard to convince old-school lispers of anything, let alone that their dialect is wrong, or needs improvement.