The vehicules are have less components thus are both cheaper upfront and cheaper on at operation by being lighter and requiring less maintainance. Because of that they are on average bigger so you can have a smaller fleet. Also installing overhead cables isn't such a big investment (in the face of related urban investment needed anyway like road maintainance, street furnitures and traffic/lanes shaping). Remember that tons of cities install christmas decorations with similar overhead wires just for one month every year.