maybe the screen writers have never bothered to read it
maybe the content is too single minded/technically focused & the remainder of the story-line too thin
(assumptions; not my opinion - i love the book, read it double figures)
'We were Soldiers' is probably as close as you are going to get to it & most likely any attempt to make a movie based on Chickenhawk would be seen as purley an attempt to capitalise on the 'success' of "We were Soldiers"
don't get me wrong - i'd love to see it done (well), with the same effort of getting it right as 'Blackhawk down'
maybe hollywood is all 'Vietnam-ed' out
perhaps they are looking for other avenues, particularly now post 9/11 & with Iraq still going, lots of people trying to stop thinking about war.
(x-military, so don't go hippy bashing me, if i was still in i know where i'd be)
in short, i don't know why & i wish it weren't so.