The main reason why electric bikes aren't mainstream is the fact that human legs and electric motors want to produce power at much different RPMs.
Casual cyclists pedal at around 60 RPM. Experienced cyclist can pedal at higher RPMs, 90 RPM. Both of these are nothing when compared to optimal electric motors that want to rotate at 10 000 RPM, if optimized to produce the largest possible power with lowest possible cost and lowest possible weight.
At 25 km/h, a wheel with 2m circumference rotates at 208 RPM.
So, pedals rotate more slowly than wheels (about 3x difference), whereas optimal electric motors want to rotate at about 50x faster than bicycle wheels.
Engineering has allowed us to create a motor producing 250 watts of power within the hub of a wheel. This is bit over what casual cyclist can produce continuously. However, even causal cyclists can climb up short hills at over 500 watts, more than what these hub motors produce.
These hub motors also are heavier than necessary due to being restricted to about 200 RPM, and also more expensive than necessary.
If you want to create a cheap lightweight electric transportation vehicle, you need to have the gearing to increase RPMs at the electric motor. This means a drivetrain that increases RPMs at the power source, rather than decreasing them. Thus, you need to get rid of the pedals because there is no way to have both the existing drivetrain for pedals and the new drivetrain for the electric motor. It won't be an electric bicycle anymore. Such a vehicle needs to be insured.
On some countries, vehicles that:
- Have pedals
- Have electric assist at up to 25 km/h
- Have at most 250 W of electric assist
- Assist only when pedaling
...can be used without insurance. They are barely adequate, but heavierweight and more expensive than necessary due to the limitation that the motor needs to be in a wheel hub, and thus rotate at suboptimal RPMs.
I believe that by eliminating the pedals, you could easily have 750 W of electric assist at up to 40 km/h, in a lighterweight package than most electric bikes, and also for a cheaper price tag than most electric bikes. However, by eliminating the pedals, it won't be an electric bike anymore.
A long time ago, there were motor vehicles with pedals, called mopeds. Then afterwards, it was realized that it is better to eliminate the pedals. The vehicles are still called mopeds even though there are no pedals. I believe a similar thing will happen with electric bikes. The pedals will be eliminated, and thus, we end up with electric scooters.