Hopefully someone with pertinent knowledge will jump in to offer an explanation for why a (hopefully) growing fleet of expensive aircraft would be allowed to remain grounded for months for want of an acceptable battery or two.
1. Corporate insanity?
2. Mountain of internal processes making the path forward glacial-speed?
3. Intense FAA review of certification found other serious non-battery concerns?
4. Battery charge/discharge electronics and software dispersed and tightly woven into the overall aircraft systems, making any change a vast undertaking?
5. Legal battles with suppliers?
6. ...?
Having spent a long career in military aerospace and watching the slide from personal responsibility to crowded meeting rooms deciding minor details, my personal guess would be (2), but since I had nothing to do with Boeing Commercial, please ignore me.