Most people will mount a tractor, horse, or motorcycle from the left side in North America for two main reasons.
One, most people are right handed and naturally lift their right leg first when climbing a ladder or stairs.
Two, North American cars are driven by entering the drivers seat from the left side of the vehicle.
Motorcycles lean to the left on a kickstand, making easier access from left side, tractors usually have more room on the left side floorboard, loader control, instrument clusters, cupholders, etc.
Some cabbed equipment only have a door access on the left side for this reason.
Is it possible to mount from the right, sure, but usually not practical and doesn't feel natural to most.