There is no single musical "tradition" ~ there are lots of them, some less ancient than others, but all consisiting of songs (and stories, and dances, etc. etc) that are commonly known by most or all members of some community.

No matter how expert one may be regarding one or more of these traditions, he/she will not necessarily be familiar with every song in some other tradition.

I'm deeply involved in several different local musical traditions here in New Orleans about which I'm sure no one else among y'all Mudcatters knows or cares a thing. So what? John the bawdy-song guy is into a tradition well-known primarily to rugby players and hash-house harriers. In the same way that many trad-jazz junkies, hashers, and/or ruggers may be unfamiliar with sea chanties and Child ballads, it only stands to reason that relatively few mainstream folkies will be familiar with the dirty lyrics to "Darktown Strutters' Ball."

Also: those who are not familiar with a given genre may or may not be interested in learning something new about a field with which they are only slighly familiar. Different strokes for different folks, right? I only open about 1/3 to 1/2 of the discussions here, because I know I won't be interested in everything. The threads I do read are about equally divided between stuff concering my most passionate interests (e.g., blues, old and new) and other discussions of topics about which I know nothing, but which intrigue and/or amuse me (e.g., Morris dancing).