Because if they left in material exchange for marks, along with the new marks for materials, it provided a method to convert Vanguard marks to Crucible marks. Bungie's intention is obviously to have those remain mutually exclusive. It would've been better to have kept quiet and have things as they were because what it boils down to is, PvE players who complained about needing to farm materials have effectively removed their access to Faction gear for themselves. A case of be careful what you wish for...