The tsar had been overthrown because people wanted peace, land, and bread. The Provisional Government kept Russia in the war. They did not engender land reform. They also did not provide the people with enough to eat. They were not a popular government.
Also, after the fall of the Romanovs, workers councils called Soviets were formed all throughout Russia. They vied with the Provisional Government for legitimate authority. There was always a problem of dual leadership. No one really knew who was in charge, but everyone knew that the Provisional Government was not doing a much better job than the tsar.