In our area we're less sophisticated, and playing in "simple" keys arises simply from the fact that 90% of the participants play guitars (and 90% of those are of course Martins "because they're louder").

A session may typically have 9 guitars, sometimes a fiddle, perhaps a couple of mandolins, and on occasion a banjo.

(Hammered dulcimers are popular in mearby "Irish" sessions, but knowledgeable players of any other kinds of music avoid those sessions, since HD players always think it's a speed contest and some of us prefer music.)

Resistance to changing keys is of course attributed to the constant need for the banjo player to "retune" whenever the key changes, should we happen to have a banjo in the band. Since we all know that tuning a banjo is pretty much fruitless anyway, we mostly pretend we're happy with the key from the preceding tune.

In actuality, some of us just play in whatever key we choose and ignore the rest of them, since with all those power-chord Martin whangers nobody else can ever be heard anyway so it doesn't much matter what we do. (We don't tell the git-whangers how much more fun we have than they do.)

Of course I'm describing a rather local "music community" in an area with little real musical tradition.

John