World War 2 was a fair-weather fight.
When the weather was bad, you didn't fight.
To that end this is a GAME, and the best most suited environment that is conducive to FIGHTING (as opposed to hiding, evading, doing stuff without detection, etc), is a clear sunny day. That also reflects most of the days fighters and bombers would engage each other in aerial combat, so it works out both ways.
EDIT: If you want a more robust cloud system that's one thing, but fog? Slamming rain? Terrible winds? Sorry, those ground planes in real life.