Here's Justin on his "Cross-Canada" touring bike. One of the problems with a non-hub design is finding the space to mount the motor and possibly a 2-stage drive. Another problem is many people (especially older folks like me) prefer a seating position that is a little more laid-back.
I recently bought a MTB because it was the cheapest way to get fat tires, suspension, and a front disc brake. However, my first choice is not the standard handle-bar/seat geometry.
By using a hard-tail with a suspension A-frame added to the back, I can have all 3 things. Recumbent seat that is lower/farther-back than stock seat. Reversed handlebar head with laid-back "moustasche" bars. plenty of frame room for non-hub motor and gear-down. From my younger motorcycle days, I much prefer the Honda Rebel seating position (shown in pic) rather than the "sport-bike" seating geometry.
I now very much like the idea of a 26" front wheel (suspension/disc-brake) with 24" rear (dirt cheap at thrift stores) added to a hard-tail
"At what age do you tell a highway its adopted?" -Steven Wright