Pre-1980s some bus companies relied on voluntary overtime working to cover not only Sundays but Saturdays as well. Typically Saturdays were paid at double-time and Sundays at time and a half. However bus companies had the flexibility of being able to recruit "weekend only" staff. Sometimes this would include bus company office staff supplementing their income and gaining front-line experience. Sometimes it was "professional" folk with a well-paid job outwith the industry during the week but a liking for bus driving at weekends. Some depots in those days sailed close to the wind in terms of drivers hours legislation by encouraging drivers to work double shifts. At times of major staff shortage often on Summer Saturdays some operators were able to subcontract a proportion of their private hire and tour commitments to local independent operators. Ultimately when staffing was particularly dire there would be cancellations of scheduled journeys often on fairly high frequency services to minimise the pain. Similar problems as recently encountered by various TOCs surfaced from time to time at times of industrial relations breakdown and this was probably the main catalyst for Saturdays and Sundays being incorporated into the week which in sectors of the bus industry was termed "five over seven".The answer to this is that it was the self same management who actively created the situation in the first place, by recruiting traincrew with Sundays outside their contracts in order to save money, and now (at least via the last government) want to have their cake and eat it by introducing committed Sundays - well that simply won’t happen!
That said, I completely appreciate the perspective you’re coming at this from as someone from outside the industry - how were Sundays treated historically on the buses, out of interest?