Merger talks between Boston Medical Center and Tufts Medical Center broke down this week, scrapping a deal that would have been the biggest alliance of two city hospitals in nearly two decades. Although they did not specify why the deal fell apart, the hospitals were apparently unable to overcome differences in culture, mission, and strategies for the future, analysts said. "Culture always trumps strategy," said Ellen Lutch Bender, president of the consulting firm Bender Strategies LLC. In a joint statement, Dr. Michael Wagner, the chief executive of Tufts, and BMC chief executive Kate Walsh said only that integrating two large teaching hospitals would have been a "complex task" and it was "best for our medical centers to remain separate," but open to future collaboration.