The biggest brokers in an area will monopolize 60%+ of all the data in a single MLS. When they have control over the listings, they have control over the MLS. If they threaten to leave an MLS, the MLS is worthless, and one company wins.
So, the MLS compromises. It does the bidding of the biggest listing offices and everyone else suffers.
I speak from first hand experience as a vendor in multiple markets. An open mls would be mind blowing and wonderful, bit it's not a matter of setting up a nice website, it's a matter of beating through the bureaucracy.