Because then you have an empty bus while it charges.

I guess it depends on how hard it is to swap out a battery but on a vehicle like a bus that can probably be made very simple. Now that I think about it a bus must carry a large quantity of fuel, I wonder how long it takes to fill one up. Swapping a battery could actually be faster.

Depends on battery charging rates but in general, a swap will be faster at large energy densities. BAIC in China are currently experimenting with this in taxis and buses.

Sure but even with low charging rates you can make up the difference with more battery packs right?