Why is the original movie so obscure?

I saw this on a CBS Friday Night Movie back around 1970. I watched it with my father (a WWII vet) who said it was the most realistic war movie he had ever seen.
Yet I have never seen it on TV since. And it's almost impossible to find a decent copy on video in any format.
Many "comprehensive" movie guides don't even list it.
I don't understand why.