I seem to be able to pick out "singer songwriter" style performances without being told what they consider themselves to be.

Folk has many sub categories and is defined for those using the definition.

For the purposes of Folk radio programming one finds that more "singer song writer" recordings are included generally than "country" Why is this so?

Much of what "singer songwriter" represents could be presented in its own programming much as country is handled today. Some crosses deeper into folk ie. sounds much more like traditional folk.

There is now enough "singer songwriter" material sounding minimally like folk and much more like "singer songwriter" to allow for distinct programming just for this genre.

Conrad