My guess would be it's P.O.V.
I suspect that, as I am sure that you have already noted the only film made recently on the topic of the Vietnam war was the turgid, and poorly crafted 'We were Soldiers..". It is much more resemblent of a WW2 movie with it's cardboard and simplistic reduction of the war to black and white and good v. evil.
As the current political polar extremes continue to manifest themselves on a daily basis during the current presidential campaign in the US, it is apparent that the divisions of the Vietnam era are still vibrantly alive and not at all resolved in the public eye.
The book, "Chickenhawk" is not only well done but also accurate in it's portrayal of life 'on the line' as a helo pilot in VN. I was there in 70-71 and also based in II Corps, and I was surprised at the similiarities far outnumbering the contrasts despite the intervening years since the timeframe the book describes.
I believe that no major studio, (for the forseeable future) with the US currently engaged in multiple conflicts in the present tense would consider making a film which wasn't ~appropriately~ 'on message'.
Sad but true. But then, as we already know... "truth is always the first casualty".