The Fallen have become like many occupying forces before them. In the initial battle they roll in with superior numbers, technology and weapons and sweep aside the enemy (in this case humanity). Now that the war is won they become an occupying force and no conventional army has ever suceeded in being an occupying force. The ease of victory breeds complacency among the occupying troops, they let their guard down and stand around talking crap while the guerrilla army (Guardians) move against them, striking at them with small and heavily armed teams before disappearing back inside their Dinklebots to wait for another opportunity to strike. This is one possible reason for The Fallen being the way they are, that or Bungie just couldn't make an AI that wasn't woefully easy to read and predict but I'm gonna go with the first one ;-)

Yeah, I asked the Speaker about that and I bet you could guess his answer. I hate that guy.