As an outsider, it's always fascinating for me to see, that most British buses only have one door. I have found buses tedious to use, when I visited the UK, for that reason (but also ticketing in general, but that's for another thread).
Contrast this to single-deck non-bendy buses in Prague, which have four doors. On the line that I mostly travelled on, most people didn't buy tickets at the driver, so even though it was a heavily-used route (two routes actually, both every 6 minutes), dwell times were very short. Doors open, quick getting off and on, doors close and off the bus went.
Now, four doors is maybe a bit too much for just any bus, but I find two doors should be the standard, with three doors on busier routes (if it's not a bendy bus or doubledecker). Single door doubledeckers are even more weird to me, since normal bendy buses (as the equivalent of the "larger bus" type) here in Germany always have three doors, sometimes even four.
Regarding some of the criticisms mentioned before, at least on bendy buses, the third (and fourth if existant) door is closed automatically, so the driver doesn't have to do that themself. Also, even while drivers sell tickets, many people have passes anyway, and tickets are checked by ticket controllers, so drivers don't (have to) deal with every single passenger's ticket in detail.
Of course that's just my opinion, and maybe there are other factors I didn't consider that explain why British buses have so few doors.
SOR NB 12 four-door bus of Veolia Praha at Depo Hostivař bus station in 2012. Image credit: ŠJů, published under the CC BY-SA 3.0 license at
https://commons.wikimedia.org/wiki/File
epo_Hostivař,_autobus_Veolia_Transport_Praha_-_1253.jpg