Because they'll just put self-interest over wider issues. This has long been the case in the EFL, where obvious changes for the better that would benefit the collective have been voted down because many of the clubs think that it might negatively impact them at some future point.
If you're not already aware of it, Google 'Tragedy of the Commons'.
Aristotle even recognised the phenomenon over 2,000 years ago "What is common to many is taken least care of, for all men have greater regard for what is their own than for what they possess in common with others."