Mounting large enough batteries for bus EVs is a challenge. Batteries are already a challenge on consumer EVs, but buses are an order of magnitude larger, and generally energy required to propel a vehicle forward is more than linear in the weight of a vehicle. Right now most implementations are in trolley buses that use batteries to compensate for portions of the line that don't have power. I imagine this will change soon but currently most buses that are not powered by gas are using hydrogen fuel cells due to energy requirements.