Public Buses in SG - Doors

I am always in the impression that the doors are designed to be at or near the centre of the bus because it cannot be fitted to the rear, probably because of the engine compartment, height, etc. 

However, if you can see from top photo below, for full low floor bus, it is actually possible. There are in fact 3 doors, the front door, the middle door and the rear doors. This bus has its physical dimension the same as SMRT bus below (Both the A22 Man - the top one being the hybrid while the SMRT's is a full diesel)

MAN A22 Hybrid Demonstrator

SMRT Man A22

And if you had observed that the situation in SG is typically this:

Read: Overcrowding

Read: Overcrowding

...and the fact that 80% of the time people refused to move in all the way to the rear of the bus, due to the fact that it is harder to alight if you are at the extreme end of the bus, why hasn't SMRT or in general bus operators in Singapore thought of building an additional rear door at the back? 

To be fair, I believe that SMRT has already requested to push the doors to as backwards as possible but there are still between the wheels, and still 2 instead of 3.

For space constraint, which I think is one of the consideration, I think that the proposed rear door does not need to be the full size as there is already a middle door. A half size (one door instead of 2 doors, just like those seen in travel coaches) should suffice. 

I believe that it will probably increase efficiency of the service, especially on those route where passengers turnaround are very quick. For example, the feeder services.