I think the answer is: sub structure. Each traditional folk song had a folklore behind it which is not true of most of the popular music from 1800's to today which is primarily composed by single known authors/composers. There are still folk cultures today throughout the world which bear little resemblance to the popular music industry, still cultivating their musical heritage away from the narrow scope of main stream media.

Songs written specifically for commercial purposes which include popular music on the top ten stations, Broadway show songs, and some jazz tunes, the last maybe not for commercial gain exclusively, since I argue that jazz overall is an outgrowth of traditional folk music associated with African-American heritage, folk songs are not written to make money.
They become popular, particularly during the folk revival of the Sixties, by accident in association with popular singing groups such as the Weavers, Kingston Trio or P.P. and M.

To enable the necessary knowledge behind each song, investigation of the folklore, cultural framework, sociology, and ethnomusicological (phew, that's big word) is necessary.

In American folk music, two main currents exist, Appalachian or so-called Anglo-American styles and just as equally important, the contribution of African-American styles, both of which are independent of their commercial value in the music marketplace.

Rock and roll is a commercial venture though some form of it will inevitably survive in a folk context, though the songs will have been changed through aural transmission, as has been the case for early popular songs that wandered into the folk domain such as Stephen Foster's "Angelina Baker" morphing into "Angeline the Baker", a hoe down tune or a song such as
"Old Dan Tucker" written for the New York stage by Dan Emmett which became a folk tune with many new verses and used as a folk dance song.

Aside from the two major influences in American folk music, there are many smaller contributions by European, Asian and Latino sub-cultures that blend with the so-called
Anglo-American and Afro-American ones.

So, the songs last because of the potent background of cultural heritage by specific pockets of folk life that still are under the radar, even today, of the commercial music business.
Corporate musical imperialism of the commercially popular song will not erase them.