good question
It comes down to the commissioner needing scale and have no way to critically evaluate if they are any good and can deliver.
Why they can't Google the trail of disasters i don't know but again most of the failures are kept well hidden.
Take Amey for example, they have recently taken to doing council waste services but did not have any capacity to do food or garden waste.
Still got the contract, guess what, there are a lot of maggot infested bins and uncollected garden waste.