It comes down to energy density.
That setup (charging where the buses go at the end of the day) requires the buses have enough battery to go their entire day on one charge. But then, that restricts the ability to take a just-now-ending-service bus that's out on the road, and redeploy it to fill in for a bus that is running late, or a bus that has broken down.
(I have seen this happen: I have seen a VTA "Rapid" 522 bus, just about to go in-service at the start of its route, switch to "Not in Service" and leave without picking anyone up. When I called in to report the weirdness, the agent on the other end checked and told me that an Express bus nearby had broken down. That was the right call: The Raid bus was closer, and the Rapid schedule frequent enough to absorb the loss.)
So ideally, you would want charging points to be available where buses sit during the day. That is at least at one end of the route. It can be done, it just takes more infrastructure.
> I assume it's already got a diesel pump of some sort to feed the buses since you don't exactly see city buses at normal gas stations.
While I was an undergrad at Ohio State, I drove 35- and 40-foot buses for CABS (the campus bus service). We did indeed have a diesel tank and pump on-site. But occasionally it would break down. When that happened, we would indeed use a normal gas station! All of the nearby gas stations with diesel pumps were evaluated for bus access.