Good question. I made a few phone calls to find out. It seems that the reason is plain chance. The boarding on the left handside was not generalized until the planes had to dock. Maybe older Fters will remember that at the time of prop planes, one could board from any side. As a result of increase in traffic, the handling on the ground had to be uniformly organized. Each piece of the puzzle had to have its own place for efficiency purposes. Likewise by the design of new airports with docking stations. Somehow the left handside was chosen as standard for boarding and the whole world followed. There seems to be no law or rules, it just happend that the left side got the better of it.