Two big differences with buses, especially modern low-floor ones:
- There's very little clearance below the bottom of the vehicle. It's practically impossible to go underneath one unless it's driven right over you. Compare this with HGVs and construction vehicles, where it's very easy to go under the rear wheels as there is often a huge gap to fall into.
- Bus drivers drive the same route every day, so they're very much more familiar with the roads and junctions. With that familiarity, they know where the pinch points are and have more time to attend to what else is going on around them. A large number of the cyclist fatalities seem to be construction vehicles (cement mixers, etc.) where it's likely that the driver is unfamiliar with the roads and his /her attention is partially on finding a way to their destination.
One other factor - while buses and cyclists share bus lanes, it's junctions, especially with vehicles turning left, that seem to be particularly hazardous. While there are conflict opportunities on a bus lane, they're not likely to be as dangerous as those at junctions. Moreover, regularly commuting cyclists are likely to know the bus routes on their journey and will be able to anticipate when buses are going to turn and avoid getting in places where buses will turn.