I'm guessing it's because the whole bartering system for Oblivion in general was quite a mess, from my experience. You couldn't really effectively haggle. Even if you had 100 Mercantile, merchants would still decline prices, even if they were the exact item worth. You were still buying items for more gold than they were actually worth and selling them for less. Aside from investing, Mercantile felt to me like a rather unrewarding skill in Oblivion.
Still, I think Bethesda should have fixed it rather than removed it entirely. It wouldn't have been that hard to do, they could have just used the Morrowind one, which worked fine for me (after the patch).