[inferiorhuman]

  If a business fails to perform well, its customers can choose another
  provider, and the business will have to improve, or else it will fail.

In this case, mayor Willie Brown (who predates the creation of SFMTA) steered the contracts for the trains and train control to Breda and Alcatel/Thales via Booz Allen Hamilton. Essentially he set up a few generations of sole source contacts for train control as you can see when the MTA was brining the Siemens cars and central subway.

https://www.youtube.com/watch?v=U6mQfWC1v98

Thing is, Alcatel did fail. Thales scooped them up. From a customer POV Alcatel/Thales failed too. They sold an end of life product that the MTA had to spend millions on just to add a new route (first part of the central subway). They've largely abandoned support of the inductive loop system.

Sure, the MTA could choose another vendor. If memory serves that's where they're stuck currently. Of course this brings out all the anti-government folks despite the main problem being a private vendor (Thales) and private consultants (BAH).