Transit Operators have a brief window of time to board waiting passengers, close the doors and continue along the route to stay on schedule. If the operator opens the door again to let you on, it could cause the bus to miss a signal, which delays everyone on board, and everyone at the next stop.
Also, if a bus has already pulled away from the curb, it might not be safe to let you on. Operators don't want passengers walking through a traffic lane. Your best bet is to be at the bus stop five minutes before you expect the bus to arrive.
