Please Exit At The Rear Of The Bus

Today, the bus stopped at a stop, and a young man said very nicely "can you open the back door please?" In fact, he said it so nicely, and even though it should have been loud enough for the bus driver to hear it, he did not get his wish. As the bus rolled forward to the next stop, the young man shook his head and moved towards the front of the bus, while ringing the bell for the next stop. He was able to finally get off the bus.

So why is it that while the MBTA encourages people to exit at the rear of the bus in order for people to board the bus in the front faster to maintain schedule, bus drivers rarely ever open the back door at each stop? Is it really that much more difficult to open up the back door as well when the front door is also open?