America has never been invaded to the level that its existence as an independent country has been threatened, at least not since the war of independence.

During WWII the UK postponed elections. We even have explicit rules for how this can be done, when required. Wartime is one of the situations where it can be required. The complexity of holding an election under wartime conditions is huge. It is also a serious distraction from actually winning the war, and so costs lives. Finally, changing leadership, mid war is risky at best. The time for a new leader to settle in is paid for in lives lost.