Let me start by saying this is not standardized thing, this you can say a convention which most plane manufacturers follow.
But if they follow something, there must be reason for that.
Well, yes there are reasons behind using left side for passengers movement (enter or exit).
#1 Pilots need to keep right side engine on as a source of power and air while passengers board or get off.
#2 Plane’s fuel doors are usually on right side, so without interfering passengers movement, plane could get refueled.
#4 Catering and baggage door are usually on right side.