Just a guess, but I think Miles non-selection might be because his role is to understudy Cotchin. So if Cotch gets injured, Miles will come in to play his role to the best of his ability. Lloyd is Caddy's replacement, Broad/Vlastuin etc etc. Hawthorn used to operate under this kind of strict role-playing regime, and while it seems pretty rigid, it does seem to work. Everyone knows their role.
Having said that, its frustrating for all when he is performing at VFL level and is continually overlooked.