I think "songs what folk sing" are what it is really about. As everyone learnt to read and write the oral tradition was bound to atrophy, because memory no longer had the "exercise" it needed, the advent of recorded music was another nail in the coffin. Numerous visionaries saw it coming and collected the songs for posterity. I think the human race, by and large, like to sing in communities, family, church, glee clubs, pub, whatever, it contributes to a "feel good factor" and in numbers the odd bum note or six isn't spotted. So, songs like Black Velvet Band and Wild Rover endured because it maybe the only song non officionados of folk know and are desperate to join in, to be included, probably a product of the decline in the oral tradition or perhaps the after glow of the oral tradition. In so far as different cultural slants go, whilst the music maybe different, although it has been pointed out that African/American styles have some resonance with Shanty singing and also it has been suggested that there may be a link with the old scottish traditional "kirk" choirs, it has to be said that the bottom line is that people like to sing together and folk songs are what folk sing whatever the genre, people will still be singing "Yesterday" in fifty years time, well at least the first few lines anyway and the officionados will still be curators of the music and the officionados will keep the music alive