The problem here is that the big clubs and their owners have spent a lot of money to get where they are, and they can't afford NOT to be at the top table each and every season. For any of the 'Big Six' to miss out on the European cash bonanza would be a disaster. So they have to ensure that it doesn't happen. They have to raise the drawbridge.

Someone earlier on said that this was down to American ownership, and to a certain extent that is true - they want a closed shop system, much like the US franchised leagues. But it also applies to anyone who has paid big to get in at the top table - they want and need to keep that position.