Several factors come into this. One is essential; the expression in the voice of the singer. Another is the obvious point that no matter what quality the voice has, it MUST be suited to the song. Bothy ballads and sea shanties are hardly operatic material! In fact many of the songs in the classes mentioned can quite regularly be sung with competence and feeling by the same singing voice and style. Some contributers to the thread may remember a television series [confined to Scotland, I think--], in which Jean Redpath, Alastair MacDonald and Kenneth McKellar formed a team to deliver Scottish folk music. The first two mentioned, as most of us know, are "trad-immersed', and very talented and popular performers. Ken McKellar has a terrific voice, and was--and is---well known for his performance of Scots song---but up till then,a repertoire which was limited by "suitability. "
Not that McKellar let himself down--"My Love is Like a Red Red Rose" and other Scottish material of that ilk was performed admirably. But what was very striking indeed was that in the aftermath, it became plain that Kenneth had LEARNED from his association with Jean and Alastair. He became, quite suddenly, able to use material which he had never before used. He modified his style of delivery where his "trained operatic' style would have done the song no service at
all. A perfect example, I'd suggest, that there are "horses for courses", but also that we can all learn from each other.