The reason for this dates back to the time before the airlines, to the maritime industry. Back in the days before there were centralized ruder units in the boats, there were steering oars and they had to be on the right-hand side of the boats. When the boat used to come in the docks, to avoid the steering oar from getting jammed, they tend to put the left side of the boat up against the keel. Later on, when the airlines started, they continued with the same method of putting the left side of the airbridge or finger.
On top of that, there are some more common reasons as well. Like before, we had the air bridges and the aircraft had to minimize the walking distance of the passengers. Since the pilot is sitting on the left side, he or she had the best view of the wing tip on the left side. So when they were to park the plane for the passengers, they parked it with left side facing the terminal so that it’s easy to park and also the distance is minimized. When the air bridges were introduced, they had to set a standard side because they couldn’t get the bridges to come from all the sides. So they kept the left side for the passengers and the right side for cargo and other stuff. The right side is used to board some passengers who are in a wheelchair because they have special lifts to carry them up.
Check out the video below for complete details: