I think ‘driver availability’ is probably a more accurate term for cancelled journeys, as at most operators it’s a combination of the long period when the normal flow of new drivers was effectively switched off, leading to a shortage, and the fact that there is a much higher level of sickness due to positive covid tests and isolating requirements.
Just how much that impacts at different operators can then come down to how many alternatives there are, either in the bus industry or others, within their catchment area.