In an interview, Mr. Peduto said a number of factors had soured his enthusiasm, including a dispute over Uber’s sponsorship of a national conference and its participation in a $50 million federal grant application.

But he said he’d also become concerned about the fate of drivers who would be displaced by self-driving cars. He said he brought up the topic with Mr. Kalanick last year. “It was to be a big conversation that never continued,” he said. “I don’t want to be the mayor who takes us back to the 1920s,” he said, referring to an era in which local steel companies ran roughshod over workers and communities.