Answer: The caliphs knew freedom of worship led to an overall stronger empire.
Freedom of worship has not been common in states of the past. However, whenever it has been implemented, it has led to stronger empires. An example is the Roman Empire. At its height, the Roman Empire controlled large areas of Europe, Africa and Asia. However, whenever they conquered a new territory, they allowed the people to continue to practice their traditional religion. The caliphs adopted a similar approach. Allowing other religions meant that the conquered population was happier, and that they rebelled less, contributing to the strength of the caliphate.
