Just a guess...but WW2 was a decisive American victory over what the world knows was an evil empire.
Vietnam was the central event that shaped and defined a tumultuous generation of Americans. It was a backdrop behind the civil rights movement, the emerging popularity of illegal narcotics, a radical youth culture, the assaniation of JFK, the moon landing, etc, and it was also the first time the US lost a war.
The Korean war had no such decisive outcome, the reasons for war and the war itself were not debated as stridently. The enemy was not so obviously evil, and the outcome was basically a stalemate. The communists were not able to conquer Korea, but the Allies were not able to destroy them either.
As for Hollywood, I guess this doesn't make for such emotionally charged subject matter, not unlike the Spanish American War, the war of 1812, the countless Indian Wars, etc.
However, I can't help but wonder if the current events in North Korea might not spark an interest in Korean war.