I honestly don't think that the bosses of provincial operators are particularly bothered about having 4 doors on a bus. Two doors would be overkill on the routes around here.
The whole dual-door discussion has been done to death on many occasions on this forum. I accept that it's useful in urban areas where
- there are frequent stops where people both board and alight
- passengers are doing short journeys and don't mind standing for a short period of time
However, when services go out of town and journey times go above a certain limit, there's an expectation that one should be able to sit down. My local route is largely picking up going into town and either dropping off at the bus station, or a stop or two beforehand; the reverse is true on the outbound journey. Seating capacity is already lower because of the need to adhere to accessibilty regulations, so adding an extra door is going to reduce that capacity further when there is absolutely no need for it.
I spent my student days in Aberdeen where dual-door vehicles were the norm. At that point there was still a distinction between "town" and "country" operators but those lines became blurred as privatisation and consolidation happened. Operators want to be able to move vehicles around as required and having single-door vehicles makes this much easier. Dual doors cost more to buy in the first place and, cost more to maintain and eventually will incur a cost to remove to rebuild when the dual-door functionality is no longer required.
Transport for London may mandate the use of dual-door vehicles, but it doesn't have to deal with the operational headache of what to do with those buses when they need to be cascaded.