They are single speed because with a load like that nobody wants to go anywhere close to the 25 km/h of legal motor assist.
I work next to a depot of those so I occasionally share a bike path with them while they are in cruise mode (moving to their delivery area, as opposed to going mailbox-to-mailbox). They go a brisk pace considering the load, but certainly not much faster than the speed required for comfortable balancing. This is no bike messenger from hell tool designed to win alleycat races (though that wouldn't exactly be the home turf of 2x11 componentry either).
Between 250W of assist for hops and acceleration and a very narrow speed band to optimize the gearing ratio for, you really don't need a multispeed transmission.
I work next to a depot of those so I occasionally share a bike path with them while they are in cruise mode (moving to their delivery area, as opposed to going mailbox-to-mailbox). They go a brisk pace considering the load, but certainly not much faster than the speed required for comfortable balancing. This is no bike messenger from hell tool designed to win alleycat races (though that wouldn't exactly be the home turf of 2x11 componentry either).
Between 250W of assist for hops and acceleration and a very narrow speed band to optimize the gearing ratio for, you really don't need a multispeed transmission.