British buses have generally traditionally only had one entrance/exit, and this originally was at the back of the bus and without doors fitted. There were experiments in having a second door on these buses, but by and large this did not have sufficient advantage in boarding times and lost seating capacity.
In the late 60s/70s, with the advent of one man buses, many urban undertakings ordered dual door vehicles. Rural and peri-urban areas generally only ever had one door buses, and really dual door vehicles were more trouble than they would be worth on those services.
In many places infrastructure was not changed to accommodate the dual door buses (with passengers alighting into the side of bus shelters, muddy grass verges and on to the road into lines of parked cars) which caused complaint from passengers (to both drivers and management), and there were several high profile trapping incidents, injuring, or even killing, passengers, and many many near misses. Drivers were expected to be keeping an eye on the centre doors, and closing them promptly after the last alighting passenger (to avoid the many instances of passengers esp. at school times attempting to board and avoiding payment) whilst collecting fares at the front. I expect overriding is also reduced if the passenger has to pass the driver they have bought a short fared ticket, from a pyschological point of view. Standing passengers/ boarding passengers moving down the bus obscured the view of the centre door mirror, which didn't give the best view anyway. Passengers often came to the front to alight, particularly during the hours of darkness, and many drivers (all of whom probably had had several near misses) would encourage passengers to alight through the front doors. (Oddly, some places were very strict and this rarely happened). Several companies, or depots of companies, had issues of drivers refusing to use the centre doors following the more major incidents involving death or serious injury.
It must be said that at the undertaking that I was employed at 1980-6 they were exclusively (aside from some open rear platform double deckers for the first year) all dual door buses in a town: but the vehicle buying policy of a small batch every year or so had resulted in five different positions and/or mechanisms to open and close the doors. It was hardly surprising that there were many near misses!
Frankly, management in general were ambivalent towards dual doors in most places - none of them wanted the unwelcome publicity of the inevitable incidents (esp. where death and serious injury was concerned) or the insurance premiums to go with them, nor the thought of the fare bilking. Drivers generally preferred to have passengers alighting under their direct view, to avoid the blame of an accident, and of course pointed to the many single door vehicles successfully plying their trade [many of them probably having worked for those companies previously]. Many passengers wanted to be set down by pavement/hard standing, didn't want to get themselves or their shopping/ umbrella etc trapped and appreciated the interaction with the driver on alighting. One also has to remember that at that time all of these vehicles were high floor, especially at the centre, and some of the steps were tall and steep and only observed by the driver through a mirror. The bulk of passengers were not equating dual door buses with faster journey times, but with unpleasant journey experience, if they thought about it at all.
Then came deregulation, and fleets were dispersed, and competitive operations started in, inter alia, the urban areas, using surplus single door buses. Passengers got used to alighting from the front door, and, apart from a few exceptions, dual door buses faded away.
So we are where we are, for historical reasons by and large. Of course there have been technological advances since to mitigate some of the issues, but the debate for dual door was lost 40 years ago.
There is little debate in this country about moving towards dual door buses, the same as there is little debate elsewhere to convert to single door buses, any more than there is debate about converting single to double deck vehicles in Europe or Australia or us to standee single deckers so favoured elsewhere. In Europe dual door buses have always been the norm (at least for the last 50+ years) and their roadside infrastructure, and roadside car parking will have been built or adapted for this. Citing that dual buses work in country X or Y does not really cut it - we are not X or Y , don't have the history of X or Y, and don't have the infrastructure either. Again, citing some stops or facilities as being suitable here does not constitute all, and mixing 'alight at the front' or 'alight at the middle' on the same route, or even the same area is going to be confusing to passengers and staff and just not going to produce the sort of slick operation so desired.
Conversion of urban services would need to be done properly all or nothing; it is hard to see where the impetus is going to come from at present.
