There are roughly a million buses, total, in the United States. Compared to auto electrification, it would take a relatively minuscule effort to replace them all with electric vehicles — that's roughly the number of vehicles that Tesla alone sells in a single year. By contrast there are about 280 million cars in the US. In general a new bus costs about ten times as much as a new car, so you're looking at a much lower expense.
In practice, it's my understanding that most municipalities are buying new buses electric anyway.
Why the discrepancy? There are a lot of reasons, but I wouldn't discount the fact that buses spend a larger fraction of their lives actually being driven, and so the fact that fueling is faster than recharging matters more for buses than cars.
Therefore even though it costs more to replace all of the cars in America, economics suggest that this will happen before the buses all convert over.
>Second, electric vehicles are currently 7.2% of all new cars sold in the USA, and only 2% of new buses sold in the USA.
You are comparing a centralized decision to a decentralized one. It is very easy to get 20% of the population to do anything; it is extremely difficult to get 90% on your side. The purchase of electric buses is a choice that can be made by governments.
In practice, it's my understanding that most municipalities are buying new buses electric anyway.