An electric motor develops maximum torque at stall or zero rotation. So as speed increases the relative amount of torque being developed feels less. In most cases it is sufficient so no gearbox is used (it would have to be STRONG to handle the peak torque).
This means at speed the performance is not optimal. As the cars get faster then a gearbox becomes more helpful to keep the speed of rotation lower.
I think a few of the road cars are now using gearboxes that have recently been developed for this specific challenge. I think I remember something about the BMW i8 as it is also a range extender with the 3cyl with a good performance envelope.
http://www.plugincars.com/gearboxes-are-coming-evs-129152.html http://insideevs.com/zf-multi-speed-transmissions-coming-soon-electric-cars/ 
