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.
Oddly enough, you can get a lithium ion car battery jump starter for less than the cost of a car battery.
The amount of effort spent reducing Lithium Ion costs has resulted in dramatic price reductions. Technologies that looked extremely promising just 10 years ago are having a hard time keeping up.
Would make sense if Tesla kept a life-time score of subjected G-forces and vibration for the pack so they could bin salvaged cells.