Similar to a lot of folks, at least in the U.S., not knowing how to drive a car with a manual transmission. What was once the norm is largely now the unknown and often feared. Pity...
It is not like tail wheel airplanes are real rare, there are lots of them still flying.
And it is not like the tail wheel airplane is some new invention that only the very skilled can master.
The regulator who sets the standard and issues instructor ratings should at least define the limits of an instructors rating, if the new instructor has never flown a tail wheel airplane then the license should be issued as valid for nose wheel airplanes only.
Then at least tail wheel airplane owners would be able to judge the skills level of a instructor when they are thinking of hiring their services.