I don't know if "traditional singers" as apposed to singers of "traditional" songs, ever put there finger in their ears."
It's an international device used by traditional singers down the ages from muezzins to broadside sellers and verified by historically by woodcuts and paintings.
It was widely used thoughout Europe and Asia - probably Bert Lloyd introduced the technique into the British revival from his work in recording Eastern European singers.
The Irish Traveller ballad seller we recorded said he used it when selling his wares at the cattle-markets in rural Kerry in the 1940s, but he said he saw his father (born c 170) singing with one hand over his ear (only eejits claim the finger was IN the ear) in pubs.
There is an excellent early woodcut of the technique being used by a street singer in Leslie Shepherd's book 'The Broadside Balld'.
Perhaps people don't use it today because they're not all that much bothered about singing in tune (near enough for folk-song maybe).
Jim Carroll