It seems to me that there are two fairly seperate problems - Cost and Laziness ! Melodeons are NOT cheap and a lot of players are not as fortunate as Lester , so cant afford three boxes (As well as the Space problem) so D / G tends to be the order of the day at a lot of melodeon led sessions .
Because of that , a LOT of musicians with Chromatic instruments get used to ONLY playing in with the D / G boys and cant be bothered to look into the possibilities of other keys .
I dabble at Guitar , Banjo , Mandolin and whistle , and refuse to pot a Capo any where near the Mando , and am trying to do the same with the Guitar and the Banjo (Except Fifth String)
Whistle is more fun , and I quite enjoy working round the keys as long as the tempo is not too fast - One of my 'fun' things is 'The Lark in the Clear Air' on a C whistle , starting in A and working through D , G , C ,F and finishing in Bflat.