There are electric versions by Boston Dynamics but they carry less weight which probably disqualifies them from the project. I'm sure someone was hoping that the firms involved would pump out an all electric version that could meet everything but it just wasn't feasible within the parameters.
I remember in one of the Ghost Recon Advanced Warfare games, there is a mini cooper sized APC looking thing that would carry supplies. It would either follow the player or could be controlled. I think that would be a better solution. A gas electric hybrid offroad vehicle would be pretty cool. It could carry supplies or serve as an automated medevac and still be silent in electric mode.