These are intersecting sets. Some, but not all, singer/songwriters are folksingers. Some, but not all, folksingers are singer/songwriters. Anyone who automatically calls singer/songwriters folksingers, or vice versa, is exhibiting extreme intellectual laziness.

The question "Why are singer/songwriters called folksingers?" is inherently fallacious if it means ALL singer/songwriters. If it means SOME singer/songwriters, the answer is simply either because they are indeed folksingers, or because whoever labeled them such doesn't care or know to make the distinction.