Leaders of some of the Massachusetts' largest hospitals failed to show up at a public hearing to answer state regulators' questions about what is driving up healthcare costs. The hearings on hospital costs are part of a three-month probe by Gov. Deval Patrick's administration that started as an investigation into the reasons for the disproportionately high health insurance rates paid by small businesses. Leaders from just two of the 17 hospitals or hospital networks invited to testify yesterday appeared at the hearing. A month earlier, officials of the state's major insurance companies testified at an earlier set of hearings, but refused to answer many key questions.