It must have become a thing over the years to not induct MK or DS
My guess is that if DS has not been inducted it is because that Mark declined.
Do you think Hall of Fame committee approach stars before inducting them? I think it's more like a honorific thing.
And why Mark isn't there I'd say because he is kind of genuinely underrated musician, which he likes anyway I think.
P.S. By "underrated" I mean exactly things like his absence in things like Hall of Fame of This and Hall of Fame of That.