The problem is that the MTA is managed by the state, not the city. De Blasio doesn't have a mandate to fix the subway; he was elected with a mandate to fix racial relations with the NYPD and get universal pre-K, and his reelection was mostly a function of there being no obvious Democrat alternative. It's why his main thing has been to promote the ferries, biking, and the BQX, because those are the only things he directly controls.

There was a gubernatorial challenger who was much more pro-subway, but she lost a campaign by basically treating it as a run for the mayor of New York and ignoring the rest of the state. (Her lack of managerial experience also didn't help.)