Has as much to do with availability as anything else I think Grahame.
Where did you get a dulcimer from back in those days? It took me nearly a year to find one, and I was really looking ... and it was crap. I ended up almost rebuilding the thing.
Joni Mitchell broke hers on tour and her Road Manager was near to desperation trying to find a replacement. There just wasn't that many instruments around.
As I remember those days, Irish bands were more interested in adding mandolins and bazoukis anyway.
The other side has more to do with what the record distribution companies put out, Steeleye Span were one of the few and Tim Hart got featured. Otherwise it was small lables like Bill Leader's that carried a few players, Roger Nicholson, Pete & Chris Coe, Stephan Sobell etc.