Have you noticed that when motorway traffic is slow, big arty lorries seem to get ahead. No matter how fast your car, lorries still gets ahead. Don't worry, I have big respect for lorry drivers and the skills it takes. But one thing must be true, that the acceleration is slow and they take up a lot of road space.
However, the driver is sitting in a high position and has good view of the road ahead. Car drivers instinctly give the lorry the space it asks for when indicating, fearing that the lorry will just move over anyway. So why not busses? Busses appears closer to cars in appearance and percieved as more car friendly, whereas lorries looks scary with multiple wheels and sharp ironwork.