Reasons were
In the 1920s, Gandhi neglected Jinnah and appealed to Muslims for a common cause, which seemed harmless at that time.In the 1930s, Nehru, assured of the backing of his socialist ideology and the support among the Muslim bases for the same, arrogantly believed that Muslims won't be moved by a party based on faith. It later turned out he was false.In the 1930s, Jinnah was willing to negotiate an agreement with the leadership, but was constantly ignored.                                                This is going over my head. Arrogance and underestimation of someone doesn't go unpunished. However blame is to be equally shared by all the top leaders. In the 1940s, ML with almost a million members had no compulsion to bow out. He had no reason to cut a deal now. He was demanding recklessly. He refused to allow first independence , then partition. I may not comment on the course of history had this been allowed.While Gandhi tried his best possible to stop the painful separation, Nehru wasn't ready to the idea of Jinnah being the first Prime Minister of the independent India till the very end. Patel hoped partition would show Jinnah the fault in his ideas and would seal his fate.