Why is mournful solemnity the intended way - we WON WW2. We lost relatively few soldiers (compared to every other major combatant at least), basically had no damage on the home front, and the effort and victory set the nation up for roughly two generations of middle class prosperity that the world might never see again.
Its not Vietnam or Korea - wars that ended in stalemates. Its not WWI where we came into the game late, and the end result was ultimately more chaotic than the situation before the war.
If they wanted a solemn memorial to WW2 dead, they should have built one. They didnt - they built a memorial celebrating the efforts of the dead and everyone else who was involved and celebrating the victory - the moment the US went from just another Western power to THE Western Power. The peak of the American Century.