I can't speak for the relative costs but adding a bigger battery adds cost, it also adds weight which increases the motor size you need, further increasing costs, it also increases wear and tear on the road.

Also the article mentioned $1.5million for infrastructure to charge 20 buses. Ideally you'd want that running 24/7, not lying dormant for 16 hours a day.

And lastly, you're adding a massive single point of failure. What if the power goes out overnight? Suddenly you don't have any buses the next day.

Ps I like the idea of buses going to sleep at night :)