In the medieval ages why did the church have power over people?

When a king had a lot of power, it was because he worked at it. Not all kings had a lot of power. Some had nearly absolute power, others were rather weak.

The history of the monarchy in the Middle Ages includes ongoing conflict among the various people and groups of people over their power bases. These groups included the kings and royalty, church and clergy, and the nobility. From time to time, they included citizens (meaning people who lived in large towns and cities) and serfs, who occasionally threw their support to one side or another in a royal conflict with important effect (as in the case of the war in England between Steven and Matilda).

The kings felt a need to defend their power. Failure to do so could mean the collapse of monarchy and civil war. So they usually worked hard at power consolidation. There were always members of the nobility who wanted to assassinate them or at least to limit their power (as in the case of the barons forcing King John to sign the Magna Carta). And the church was always looking out for its interests (at one point the church owned a third of the land in England).