Question

why did the Persians agree to make a deal with the Arabs

Answer 1

Persians of Iran decided to make a deal with their Arab Neighbours in order to stop the military conflicts and political disturbances in the Middle East region.   Persians and Arabs signed an agreement in 1975 over non interference in each other's political matters.   They also settled the land frontier and water frontier disputes to maintain peace in the region.

Answer 2

The earlier situation was that the Arabs identified with the Persians by writing and speaking Persian.

However, when Islam, as a religion began to gain strength and the Arabs, conquered the Persians.

Then they also started identifying with the Arabs and started writing in Arabic along with Persian.

This is how the Persians joined hands with the Arabs.