Bus and MAX operators have a brief window of time to board waiting passengers, close the doors and make it through a green light. If the operator opens the door again to let you on, it could cause your bus or train to miss a signal, which delays everyone on board, and everyone at the next stop, and… well, you get the picture!
Also, if a bus has already pulled away from the curb, it might not be safe to let you on. Operators don't want you walking through a traffic lane.
By the way, we don't advise running for any bus or train… If you have to run for it, wait for the next one. It's not worth the risk!