Because I am always on the left side of the bike when I want to release it. You generally don't stand on the right side of the bike to do anything because you will get lubricant from the chain or the rings scratching your skin or tearing your clothes.

I also think there is some merit to having quick releases toward the open side of the road. You will swerve to the left to avoid debris so the quick release will be on the side of the bike that is away from the hazard. The same thing with bushes, etc that are along the side of the road.