Apples and oranges. These electric cars you speak of that travel highways use battery packs weighing 650-1200+ lbs and are physically larger than the cars used at WDW. And that's just the battery, never mind the electric motor and the associated running gear. Even then, the smaller ones...such as the Nissan Leaf, can only go around 100 miles. Keep in mind the type of abuse the Tomorrowland Speeday cars are subjected to, daily. I don't know the specific details of the miles put on a Tomorrowland car, but I'd wager that it's just not realistic to have an all electric fleet that would have the capacity to move as many guests as the current set up can. My boys love to race electric go-karts. I just took them there yesterday...those carts are plugged back into a quick charger after each and every 6-10 minute race. Yes, they use more electricity than the WDW cars going 8mph or whatever they go, but still I just don't know they'd be able to do it to full scale. WDW would need hundreds and hundreds of them and have to constantly rotate them out, because they couldn't take a chance on having one run out of battery halfway around the track. They're also temperature sensitive, lose capacity over time, can be sensitive if not cycled properly, etc...they're just not as easy to maintain as a simple gas lawnmower engine.
So while it would certainly be nice if they were electric, not sure that's plausible right now. The technology may not be quite what it needs to be yet. Getting newer, more efficient gas engines, sure.