Because players were to lazy to learn how to collect mats and complained. Bungie gave them a way to buy the mats now, but Bungie didn't want a currency exchange system ie buy mats for Vanguard Marks and selling them for Crucible Marks.
English
-
Still a pointless move. It costs ten marks for 20 materials, but it used to cost [b]50[/b] materials for only 5 marks. So if you spent 50 Vanguard marks, half your weekly limit, you would only be able to get 10 Crucible Marks. That's a 5:1 exchange rate.