The answer to this is simple, they are protecting their interests.  One of the most difficult part about legalizing MMJ Federally is that we need major money and lobbyists to go toe to toe with the established lobbyists that already have a foot hold.  That's who get's their agenda driven through.  My belief is it is as simple as political support, Obama took the political support of whoever wanted MMJ to stay as is.  That's why he is clearly avoiding it IMO.