Not only is there the startup /development time but there are the unforseen bugs which arise only when the tech is deployed across millions of units in the real world rather than just the lab. Lithium tech is mature and getting better, though clearly it is not perfect (fires, long charging times).
Still "..lithium-ion batteries keep getting better. Energy density is increasing 30 percent every two or three years, he told
C/D, while costs fall significantly." - see
BMW Ponders Batteries for 2021 EVs, Says Solid State Isn’t Ready Lithium's great advantage is, "I got there first" (shows the finger)..
I suspect that when we are talking about the sort of energy densities we need these batteries to have (propel a 1+ ton vehicle ~1000 km over typical terrain on a single charge), fires might be a problem whatever the battery type.
There clearly is a big race to crack this nut, develop a battery which will do as indicated above and have it recharge in < 5 min, for tens of thousands of cycles with < 10 % degradation e.g.
Solid-state Battery Breakthrough Promises One Minute Charge Time
