If you double the speed, the air resistance gets quadrupled. Power output needed for constant speed travel is octupled (P=Fv). Now you need a higher-power motor and more expensive batteries that can sustain this kind of power output. The typical electric vehicle available today can't sustain a speed of 100 mph without thermal throttling kicking in.

Sure making the electric motor more powerful isn't that big of a problem: just look at electric trains. But making the batteries more powerful will require significant more research.