Because their objectives are different but what they all want often meets. The clubs at the bottom want to be able to invest and stay in the PL; the clubs in the middle don't want the bottom clubs investing and catching them up with them; the clubs below City either want to invest to catch up to City, or just want to cripple City so they can catch up without investing (in either case, they don't want any newcomers investing and overtaking them). So you get turkeys voting for more food, but then Christmas comes along.
There is nothing wrong with a two thirds majority, of course. The problem lies in which votes are put before the meetings in the first place. Someone needs to be filtering out proposals for votes which are not in the interest of the league in the long term. Proposals which are legally suspect, for one, and proposals that restrict investment, for another. It's Masters' job. And he is piss awful at it.