From the main Scheme article on Wikipedia: "Scheme follows a minimalist design philosophy specifying a small standard core with powerful tools for language extension. Its compactness and elegance have made it popular with educators, language designers, programmers, implementors, and hobbyists."

Scheme is widely (mostly?) used as a teaching and research language in Comp. Sci. schools. This sort of environment encourages experimentation with the language itself, whereas Python and Ruby are more oriented towards practitioners who are more interested in using the language as-is, not experimenting with it.