Why they flock to him? It is simple. They believe he is the Messiah.
The Jews back then read the Bible heavily (and they still do today.) But if noticing a common trait, there are only temporal moments of peace in the history of the Bible due to figures like Solomon, David, Esther and Samuel. However, prophetic scriptures from Isaiah, Joel and other minor prophets speak of oppourtunities of eternal grace from the coming Messiah. They believe he is coming down with coertion, force and divine judgment upon thy enemies. Instead, he uses the greatest weapon of all: grace. It is so merciful yet powerful, meritless yet violent. It says it is like feeding your enemies with hot coal. From Jesus's application of grace and unconditional love, people realize that he was the Messiah and so therefore, flocked toward him.