Flying in Europe, it is about a 50/50 chance as to whether you get on the plane from a jet bridge or you take a bus to the plane parked on the tarmac. In contrast, most US airports have jet bridges, even when the plane is small unless it is a very small airport.
Why?
Nah, I worked at an airport that only has CRJs and we had a jet bridge. I’ve even boarded Saab 340s from them. There are some differences like a little bridge instead of direct connection, and they can’t use the jet bridge’s auto-level feature (as the plane gets loaded and unloaded it slowly sinks/rises not unlike what a car would do; auto-level has a stick with a wheel to track that and move the jet bridge to match) but other than that it’s pretty seamless.
Ok, there is a limit and you probably couldn’t use a jet bridge on, say, a Cessna Caravan… But the minimum is most definitely not a mainline plane.
How about a dash 8? They’re pretty common around here, and I’m having a hard time imagining how one would connect a jet bridge to them.