My guess would be that cost and safety are the reasons. The cars are on the track running the whole day, so there is no way any battery could do. So they either would have to recharge each car very regularly or they would have to implement a way to get the electricity to the cars while they are driving. An electric rail or something like that on the middle of the street would probably cost a lot and it would be really dangerous. And you wouldn't want something similar to what those carnival bumper cars are using, would you?
