No reason why it can’t all be in a single “campaign” with proper progression and match making - no reason at all… except money and effort.
Similarly North Africa could be one that segues into Italy, which segues into Normandy and northern Europe
Heck even within the existing “campaigns” the is room for good logical progression - Barbarossa-Minsk-Smolensk-Moscow-Rhzev for example, or Cyrenaica-Crusader-el Alamein-Tunisia… or Bagration to Berlin…