They had to have a battery that would function in both extremely hot and extremely cold weather. Lithium batteries shouldn't be charged in very cold weather. They did cheap out a bit with these batteries, but the chemistry makes some sense.
I know of only one EV manufacturer who does not use lead acid, and that is a recent change. Tesla changed to LFP due to vampire drain from all the awake electronics. If that is not the case, certain formulations of lead acid work well and is cheap.
