THE WORLD, THE FLESH AND THE DEVIL: message gets lost in the mystery

Made in 1959, during the height of the Cold War, THE WORLD, THE FLESH AND THE DEVIL exploits the very real fear of annihilation by nuclear attack to explore issues of racism and humanity, but they're unfortunately overshadowed by the mushroom cloud-sized nagging question of what exactly has happened to humanity.

African-American mining engineer Ralph Burton (Harry Belafonte) emerges after five days trapped in a collapsed tunnel in Pennsylvania to discover he is apparently the only person left alive. Everywhere is completely deserted, homes and vehicles have been abandoned in what looks like a mass panic. Heading east he makes his way to New York City searching for signs of life but all he finds are eeriely empty and silent streets and skyscrapers. 
From what he can piece together from newspapers and radio recordings, life on earth has been wiped out by a deadly radiation cloud released by an unnamed enemy. Then a blonde haired, attractive young woman suddenly appears. Sarah Crandall (Inger Stevens) has also escaped the poison cloud.
With this mismatched and reluctant couple thrown together by fate, the question is can they  overcome their prejudices and preconceptions to make a life for themselves? Can they even bring themselves to touch each other? 
This was 1959 remember; the rise of Martin Luther King and the Civil Rights movement to national prominence was still several years away, Belafonte and Sidney Poiter had only recently emerged as cinema's first Black leading men, and the idea of a Black man kissing a white woman on screen was still beyond the pale. The film makes a fairly half-hearted attempt to explore these taboos but our attention is distracted with wondering just what has happened to all the people.
New York is a city of eight million souls and it's clear from the enormous jams of abandoned cars that most did not make it beyond the George Washington Bridge, yet there's not a single body in sight. All these millions and millions of people have just vanished. It defies logic and science. Yes, some of those closest to a nuclear blast will be vaporised but as the experience of Hiroshima and Nagasaki showed, tens of thousands more died slow, painful and visible deaths.
And anyway, according to THE WORLD, THE FLESH AND THE DEVIL (hereafter referred to as TWTFATD) it was a radiation cloud not a bomb which wiped out the human race, so it's highly unlikely to have also made every last body vanish. 
While I'm a sucker for these 'last man on Earth' movies, mostly because I love the idea of having a completely intact Manhattan entirely to myself, this glaring lack of corpses stinking up the streets seriously weakens the film's efforts to discuss loftier concepts.
Belafonte is an impressive screen presence and a fine actor - his second release of 1959 "Odds Against Tomorrow" is one my favourite films of the 50s - but as producer of TWTFATD as well as the star he lets himself down by sanctioning this enormous hole in the plot.
And as for the ending - please don't get me started! All I'll say is if there was an Oscar for the year's most implausible, biggest cop-out finale it would be known as the TWTFATD.