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?
A lot of budget Airlines (in Europe: Ryanair, EasyJet etc) try to cut costs of airport service and one way they do that is to accept bus transport or walk out to board.