Generally speaking since DC charging is so fast, you need to have very tight control over the cooling and performance of the cells or you’ll have issues with longevity. Lots of retrofits just kind of stuff batteries wherever they fit… so cooling isn’t going to be the most effective. By comparison, AC charging, even at 12kW, is quite slow for a decent sized battery pack.

I don’t think there’s any technical/protocol reason you couldn’t do this, it just complicates things quite a bit.