The reason seems pretty obvious to me.
A municipal operator is a monopoly. It hands far too much power to the unions.
Look at Khan, allegedly the most far left Labour politician out there with bus powers. Talk of taking back control of London's buses is just that. He's happy to persist with route tendering even as the pool of bidders has dwindled to a handful.
I'm quite sure by now, there are effectively no benefits to London route tendering, once you factor in the costs of managing the bureacracy required. And yet for some bizarre reason, it was the largest London operator that was the only new entrant to the Manchester scene when franchised. Why? Burnham had no other choice. Who else knows how to navigate a "London style" bidding process, other than those who are cleaning up out of being the middle man between London bus drivers and the London Mayor's bus managers? All under the watchful eye of a man whose father drove buses as a direct employee of the municipality of Londinium.
There is also clearly a lingering fondness within Labour for the principle of competition driving down costs. They still do not accept the argument that the only way an employee providing a public service will truly serve that public to the very best of themselves, is if they are quite literally being directed and rewarded by the duly elected representatives of that public.
All the other reasons don't really wash. They are excuses, easily overcome. The power of a single union holding all the cards in a bus strike, not so much.
it's not an accident I don't think, that the least known aspect of Mayor Burnham's bus network, now Prime Minister Burnham, is that his bus drivers are still employees of private multinational transport conglomerates. As they have been since the patchwork of privatised operators, many actually owned by their employees in part, were hoovered up into a handful of big groups.