My introduction to the idea of poly came from reading old Heinlein novels as a kid (it wasn't called poly then) and a fascination with folkrock and the historical1960s counter-culture movements and writings of that period.
I have only recently come to appreciate some sub-genres of EDM and only have been to a few concerts = probably the first was Infected Mushroom at Burning Man 2016? We've recently been to some breakbeat shows with some friends of ours who are aging ravers. (We went to a lot of industrial techno shows in the 90s but I don't know that EDM was a thing then.) Not aware of PLUR (I am older and my tastes run to music that is older than I am) but I can see how that would fit.
I think that probably what you are observing is that people who consider themselves part of any fringe, or non-mainstream, group - SCA, gamer geeks, alt music fans, burners, festival kids, etc. - are going to be more open to other "alternative" ideas...like poly/ENM. There are many paths to poly and many ways to practice ENM (poly, open, swinging, etc.) - that may be influenced by who/where you had your first exposure to the idea.
Jane(Old-Timer)Q