I believe at the time all commercial aircraft in the USA, over 80,000 lbs max weight, had to have a flight engineer.
This is why the BAC 1-11 series 400 that American Airlines bought was certified at 80,000 lbs MTOW, and thus two crew, rather than the Identical 300 series which was 87,000 lbs MTOW.
The rules must have changed when the Boeing 737 and DC9 came along