The main rationale is finance. (see also the proposed major cuts to underground staff)
One of Mayor Johnson's last acts was to cook up a deal with Chancellor Osborne to remove national government funding for TFL, which as I understand it was previously justified in part on the grounds that plenty of people who are not London residents also use TFL services (commuters and visitors from the home counties, tourists from all round the world)
Had London elected what the tories considered the right colour mayor (or right colour rosetted mayor) then no doubt one of the first acts of the new mayor would have been to strike a 'new deal' with whitehall.
Covid has seriously buggered TFL's finances, and while emergency government money has been thorn at private sector bus and train operators with few strings, TFL is under pressure to get back to a 'sustainable' / break even position.
It is probably fair to say that bus passenger numbers are still nowhere near what they were pre-covid, especially when it comes to central london commuters getting from main line station to office. Whether now is quite the right time to make long term plans based on current use while government is still pushing a 'go back to work' line to people who are working from home, is another question - when infrastructure (bus garages, bits of railway line / station) is closed and demolished / sold off / built over, it gets expensive to get that capacity back.