Here in Birmingham (where all buses have manually operated ramps),
passengers tend to be very centralised in their destinations. Between
my house and the centre (about 30 stops), only about 2 of those are
major points for getting off. In the city centre, everyone gets on and
off at one stop*. Therefore, the bus is pretty much like a bucket
filling up with water, and emptying at the last minute. If two doors
are provided, the chances are loading/unloading times will not be
increased.

In London on the other hand, I can picture buses being a bucket of
water with a hole in the bottom. Passengers will get on everywhere,
and off everywhere (tube stations, railway stations, bus interchanges,
etc). Therefore it helps to have the extra door to improve efficiency.
London is very decentralised, Birmingham is not, therefore those extra
6 seats that we can fit in on our Birmingham buses are more cost
effective than an extra door.

(*Timing point so buses tend to wait for a while anyway.)