Peter Kennedy didn't ask for D/G melodeons on a whim, he got them so that melodeon players could join in sessions which were already in D and G.

A D/G melodeon is pitched rather high, and I think most English players would actually prefer it if our standard instrument was in other lower-pitched keys, as they sound a lot better. Instead, we have been forced to adopt D/G as standard in order to accommodate fiddlers, and then we get the blame for everything being in those keys!