I think it is a mix of many parameters and the main one was that when cx was invented people were using toe clips with straps, albeit looser than on the road and they would hit the ground until the rider has put his foot inside when the bottom bracket was too low.

The rest is mostly the ability to pedal in the corners without clipping a wheel, which help maintain and feel grip, tradition and also to help on the many off camber sections the courses used to have.

I think quicker turning has never been the true reason imo.