It's drivers, not buses that is the shortage.
Most bus operators are struggling to run their regular services at the moment, and the drivers who want to do overtime / rest day work are working as many hours as they want to keep the regular services going, and don't have the need or the driving hours left to go and do rail replacement work at weekends.
As with most things in life, there's a variety of reasons behind this, including -
post-Brexit referendum, a proportion of drivers from EU countries have 'gone home';
the supply of new drivers was disrupted because training / tests more or less stopped during lockdown;
the time it takes to get a provisional PCV licence and test means that people who need to get a job (rather than people who want to change job) often give up during the wait and go off to take a job where they can start straight away;
the demand for delivery van drivers on similar sort of pay increased significantly during lockdown, and some existing PCV drivers - as well as potential PCV drivers who can do the job on a car licence (and therefore start immediately) - have gone in to that line of work;
a proportion of part time drivers have not kept their PCV licence up, again partly a product of lockdown and work stopping, which will have prompted some drivers to go from semi retired to fully retired (bearing in mind that drivers aged 45+ need to renew and do a medical every 5 years, and drivers over 60 (or may be 65?) need to renew with a medical every year - and then there's the annual CPC day. In theory that's tax deductible if your employer doesn't pay for it, but if you only work part time and don't quite earn enough to pay income tax, that doesn't help...)