Interesting question....  It happens that many of my favourite composers
also draw from their folk tradition, - Vaughan Wiliams, Sibelius, Holst,
Delius, etc., etc., and it works for them, as it does, say, for Copland.
Maybe an answer lies in the fact that a great many American folk tunes
originated in Europe and can be found in both locations.  Could it be
that a composer might then have a sympathetic feel for a particular idiom,
coming him(her)self from that cultural strain, whereas it is eminently
possible to come from another strain or tradition, and despite being told
that this is "your" folk music, find that it seems alien to you.  I give
up...  I really don't know the answer, but it's interesting to speculate
none the less.

Keith.