...while included realms which depended on foreign non-German powers like Denmark (Holstein), UK (Hannover) and the Netherlands (Luxemburg and Limburg)?
I have read in many sources that the reason for excluding Hungary and Eastern Prussia was that 'they did never belong to the HRE', but it seems a quite stupid reason IMO.
In the case of Hungary it could be argued that they were not mostly populated by Germans, but Bohemia and Carniola were already included; Eastern Prussia was mostly German by then.
Following the HRE criteria Belgium should have been included too.