Look up deir yassin. That's why they left. They also left because the creation of Israel was a United Nations Assembly vote which is non binding, that means that it has no force under international law and has to be agreed by all of the interested parties, such as the Jews that had immigrated to Palestine and the Arabs who already lived there.

The Arabs were opposed to giving up their land to a load of people that had been mistreated by Europeans. And why should they, they had already been promised the land by the British in support of their actions in the first world war.