Answer:
They needed the income from taxes assessed on non-Muslims.
Step-by-step explanation:
The main reason why the Abbasid dynasty encouraged, but did not force, non-Muslims to convert to Islam was because they needed the income from taxes assessed on non-Muslims. Therefore, their reasoning was purely economical. If the Abbasid empire became culturally and religiously homogeneous, they would not be able to enjoy the economic benefits that a diverse empire gave them.