They only care about the loading unloading time per bus stop.
Three door for double decker bus reduce the timing to unload passengers significantly. Especially for bus stop with a lot of passenger disembark like schools or mrt station.
In Europe, dwell time in bus stops are very short, other than the single decks come with 3 doors and their articulated buses are 5 doors, they are all doors boarding/alighting. Double deckers are relics in left hand drive Europe, simply because articulated buses are way more efficient, carries more passengers and lower dwell time.
True 3 door double deckers exists in very limited cities like Berlin. These buses are 14m long, low floor, no headroom issues, 1.2m door aperture.
In sg, we have old British road and construction Road Traffic Act, which states vehicles are to be max 12m. This resulted in the 3rd door of our double deck to be smaller than 1.2m. Funny thing is even the Brits discarded their old road traffic act for UNECE.