Left front didn't work well for Cyclo Cross dismounts.
A left side dismount called for a smooth left handed rear brake modulation, swing off and raise bike to right shoulder w/ right hand and run.
A decelerating rear brake on loose/wet surfaces is predictable while a front brake pitches the machine up as the rider's weight transfers from saddle to left pedal to ground.
Getting whacked in the head by one's saddle on every dismount is considered both inelegant and slow.

-Bandera