Armies don't typically fight unarmed opponents in close quarters. If you're unarmed and the attacker is within a few yards of you, a knife or a gun are not that different.
I'm pretty sure they were waiting until the Supreme Court composition favored them before bringing a case. Heller was a 5-4 decision, filing when the court was more left/liberal could have had resulted in a devastating judicial precedent for gun owners.
http://www.youtube.com/watch?v=9igSoJHEdUo