I was a MM in there for 25 years. Back in the day it was the only game in town for indexes. The SPX was a much smaller product and more institutional. There were probably 400 people in the OEX pit at one time, the largest on the CBOE floor. All the other indexes came later.
People were really getting burned on the early exercise (for me it created a lot of opportunities). There were a lot of requests from firms, retail traders and MM's to do away with the early exercise, which they almost did. Instead they compromised and created the XEO. The majority of the volume stayed with the OEX.
People still sold boxes, they just traded with a premium because of the exercise risk. We had until 3:20:59 ct to exercise options, but for public customers this wasn't a practical deadline, as the firm would need to exercise by this time, so it would be much earlier for them, which put customers at a real disadvantage.