What key a person sings in definetly depends on the melody and the vocal range required in a tune. If a melody doesn't span much range it can be placed in the upper end of a persons vocal register (read higher key) and still be doable. But, many traditional folk songs don't demand huge range that require vocal training.

Many men tend to sing in a similar range of keys like F,G,A,B. Men that tend to sing in the keys of C,D,E either have higher than normal tenor voices, or lower bass voices. Many women tend to sing in C,D,E,F. In Bluegrass music for example, Bill Monroe had a higher than usual male tenor voice and often sang in Bb and B. Thus, Bluegrass vocal styists and Bill Monroe devotees tend to sing in the upper range of their voice to match his "high tenor sound".