Cause the noobs would of bitched and cried. And since activision makes games only for the casual market, they told bungie fix that shit or face legal lawsuit action. Come on everything that is done in this game is made and done by threats by activision. Speculation mostly, but all you have do look at how activision does business: gutting games to the bare minimum, suing people or businesses, firing people or acting like the mob. Plus, its not hard to connect the dots on who gives the orders on the game itself.

They used stack if I remember right. Bungie stopped it though.

Ammo bonuses don't stack

Seriously, there's nothing in game to indicate they don't. And guns can have stacking perks, so I don't see why armor can't.