The stop-start nature of city busses makes them a real low hanging fruit for battery electrification, benefiting from instant torque to start and regen to stop and, as you say, fixed known routes within larger fleets.
The only nation that seems to have capitalised on this basic fact is China which bootstrapped its EV industry on busses, pulling ahead from 2010 and hitting 90% of global market share for EV busses in 2020, and now a big exporter.
For the same reason the electric mail trucks are a good idea. You can probably expect UPS and FedEx to start replacing their fleets over time as the existing vehicles age out, now that all electric vans are starting to become available.
I'd say 95% of Amazon delivery vehicles I see in NYC are the electric type. Amazon are known to be really concerned with the bottom-line efficiency, so I suppose it must make clear economic sense.
(At least in King County Metro) the newer diesel-electric buses are series hybrids that use electric motors for traction, and diesel generators to power a small battery. So the low hanging fruit of electric traction was already “picked”. You can look up the bus model online - New Flyer XDE class
The only nation that seems to have capitalised on this basic fact is China which bootstrapped its EV industry on busses, pulling ahead from 2010 and hitting 90% of global market share for EV busses in 2020, and now a big exporter.