Because of disagreements of the usage of portraits of Jesus, Virgin Mary, and the saints int he churches.
Explanation:
The Christianity started as a unitary religion and it was like that for few centuries. That didn't lasted long though, and soon the two main Christian centers, Rome and Constantinople, entered into a disagreement over the usage of representations the holy figures of the religion.
Constantinople was allowing paintings of Jesus, Virgin Mary, and the saints to be put in the churches or exposed publicly. Rome was not fond of this at all, and it was prohibiting such acts as it saw them as inappropriate.
Officially this became the theme that separated the two sides and resulted in the formation of the Catholic Church and the Orthodox Church. It has to be taken in account though that another major reason for the split was having more power, as the two cities were competing against each other as to which one is the center of power.
