These are the factors:
B10M
Mk2s: Easy to maintain, but O.405s even more easier.
Mk3s, Mk4s: Maintenance wise, O.405s still easier to maintain
Relatively few operators chose these chassis as city bus as
compared to operators who use them as coaches.
Scania N113CRBs: Harder to maintain than O.405s.
O.405s: Widely used chassis around the world (besides North
America). From NE England to Israel to Kuala Lumpur to Australia,
you can find these. So they need not find the parts for an O.405.
Drivers seem to like O.405s than new Volvos or Scanias. You can buy
a lot of new buses, but drivers would still stick to O.305s and
O.405s.