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?
Really depends on the airline. Lufthansa will rarely send you on a bus. But the answer is because they don’t have enough gates for all the planes.