Take a look at ggm's comment -- the machine wasn't designed for power storage, it was designed for short bursts of frequency regulation.

When designing for grid-scale capacity, there are companies looking into flow batteries and chemistries that use cheaper (non-lithium) materials.

Tesla however built out tons of lithium manufacturing capacity for their cars, and given the design constraints of this project, their existing products were a good enough fit for the job at hand.