The reason is simple. If they left in the ability to trade materials for marks, people would use it as a way to convert vanguard marks to crucible marks or vice versa. Think about it? You trade a bunch of vanguard marks for materials, then trade those materials back for crucible marks. It eliminates the need to play their god awful crucible mode. It also lets people play exactly how they want to play, so of course bungie can't allow that...