Richard,
You must be hopping mad about this!
I think if I remember rightly George Frampton has been addressing this point recently. One answer of course is that the early 20thc collectors all lived much to the west of Kent. Sussex right across to Cornwall all got a fair trawling, but some counties further north also avoided the sweep. My native Yorkshire wasn't that well covered as Kidson didn't actually go out into the field much. He just had a few mates and relatives who happened to know songs and other singers.

Steve