Imaging Resource interviewed a bunch of Canon managers and engeneers:
About the 20mm flange distance: They said that their first consideration in choosing the flange structure was the physical ruggedness of the system, and they felt that 20mm was what they needed to meet their goals for overall strength.
RF lenses superior to EF lenses: They talked a lot about how much having a large element close to the sensor helped when it comes to correcting aberrations.
Overall the focus of this system was on autofocus performance and optical lens quality.
via CanonWatch.