Because belonging to the church (being a church official such as a priest or a bishop) esentially meant you were given education (not common or at all present in those times) and a good position with a pay. You also exerted a lot of power in your local or regional community as church officials were considered to have been influential figures in medieval ages.
The Jewish people, since they appear in the history seem to have lot of problems with the other people that lived around them, often being persecuted or enslaved.
Initially they have been enslaved by the Egyptians and were not in nice relations with them, saving themselves by the scratch, but ending up in a desert. After they settled in what is now Israel, they have been conquered by the Assyrians, and since they were rebellious, the Assyrians systemically misplaced them out of their homeland. After that came the Romans, and it was similar, as part of them was again moved forcefully in other places of the empire. The Muslim Caliphates showed no mercy toward them, and they were terrorized and forced to move away. Then they had troubles in Europe as well, especially with the Spanish inquisition where they were burned alive publicly or killed without any mercy.
Government became necessary in order to rule the people
It was a tradition economy.
