score:3
The likely reason seatbelt usage is not common on buses is that buses are extremely safe as is. For instance this US study showed buses are many times safer than driving and that intercity buses are especially safe even for non-passengers who might be involved in crashes with them.
None the less, seatbelt usage in intercity buses is gradually becoming required across EU countries, the US and Canada. Mostly starting with requirements that all new buses be outfitted with seatbelts. Still it seems relatively uncommon that passengers actually use these and enforcement would be challenging without sending out undercover officers - which frankly seems like a lot of effort for something that is a relatively small risk vs. violations of people travelling by car or truck. However, I've personally seen people get scolded by other passengers for not using seatbelts in Sweden.