Transport is at the top or near the top of the agenda of mayors of any significant city. It has constantly been at the top of the agenda for the Mayor of London, despite the existence of other huge issues such the Olympics, housing, policing and terrorism. So a mayor without power to change transport is not much of a mayor at all. This probably dawned on the Tory party at a late stage, but by that point it was too late to stop and in any case, they have so little interest in buses that the end of deregulation was a small price to pay in the grand scheme of things.
You are slightly missing my point; The Tories are not alone in having "little interest in buses". By the same token, "Transport" is near the top of the Agenda for Mayors/Candidates, but by "Transport" they invariably mean Motoring and Rail (and increasingly, Cycling). Just as the Media are only interested in those three forms of Transport.