Directors probably don't like movies with pre-established backstories that they have to obey. They also figured that after A:R and the complaints about the reimagined Battlestar Galactica, where people were criticizing it just on its premise, that there was no way to please the fans. The budget probably hurt too, since it meant limited special effects and the possibly of getting a sub par writer.
However, they could have done it if they had the movie set in Iraq or something during the war on terror.