from what I gather (not an implementor) r6rs was too much to implement for the smaller 'testbed' scheme's popular with language designers, and perhaps a bit too

from what I gather (not an implementor) r6rs was too much to implement for the smaller 'testbed' scheme's popular with language designers, and perhaps a bit too proscriptive for the core - which is why r7 has the small/big split. small can be implemented more easily, and big can be added on as a portable library or alternately implemented at a more low level by the language runtime if desired