Why are buses exempt? Too cold for electric buses?

Oslo has six electric buses in use right now, and will add 70 more by next summer so electric is absolutely an option, but this is a pretty recent development. Much like Denmark none of the major parties here want to actually enforce the 2025 "ban" on fossil fuel cars either, so we can't really expect anything to happen to buses either (at the national level). On the local level the public transit in the Oslo metro area is planning to be fossil fuel free in 2020 and zero emission in 2028.

Not sure, probably the fact that they need to run all day without stopping to charge.

We have plenty of hybrid buses where I live though (in Norway).