It is choice.....convention has left wrist as it doesn't the get int the way of stuff most people's dominant hand does e.g.writing so you can check the time whilst doing something else / less likely to damage it??
If you remember the old days when you had to wind the watch up, it was far easier to do on left wrist as the winder / adjustment button is on the right side of the watch as standard.
I'm left handed, and tried wearing my watch on the right - that felt wrong, so I wear on the left wrist.
Very boring answer