Because making it work is difficult and energy density is worse. But sodium-ion will eventually be part of the story. However it will not be 'switching' both will need to be growing fast at the same time.

I think it's because sodium ions are larger, so they diffuse more slowly in materials. But maybe that doesn't matter as much for longer duration stationary batteries where lower power density is tolerable.

There’s few reasons not to, and eventually I think people will realize this.