Quote from Jerkstore:
The SPX is the last bastion of open outcry at the CBOE. The SPX is the only pit at the CBOE that still uses the old school method of disseminating only one MM company's quotes at any given time. There are about 5 different firms that rotate cycles to drive the wide screen markets. It's a painfully antiquated system that they haven't changed because it is still a singly listed product where the exchange and all MM's are making a killing. Noone wants to kill their golden goose.
As soon as the SPX is listed at the all electronic C2, this should change quickly. In the meantime, trade CME ES options or SPY options.