Why did the Arab Conquests succeed?

The success of the Arab Conquests can be easily broken down into two factors: the strengths of Islam and the weaknesses of their enemies. This can be seen in the worn down attitudes of the Byzantine and Sasanian Empires, who were weakened from fighting each other for over 25 years, not only this but Plague of Justinian had also taken its toll on these two empires - in fact the Sasnian Empire itself was completely destroyed over this time. These failings had a profound effect on the surrounding Christians, who saw such losses as retribution for their own sins and so decided to fight back and who also saw how weak their Byzantine and Sasanian leaders truly were (this was especially true for the Copts in Egypt and the Nestorians in Iraq) and all of which cultivated into a Christian society who were not fighting back, but who were also willing to give a new religion and a new leader chance to take over and to guide them on a right and better path.
Though their enemies were weak, one should not discredit the tactics and strengths that Islam did possess. To begin with the call of Islam was a rally cry of religious zeal for many in Arabian countries, as Watt argues it couldn't have solely been money that held them all together as it would have eventually caused infighting and fractures, therefore, they had to have been held together by something stronger - a common religious belief. Additionally, all these victories that the Muslims had would have had a psychological effect on their opponents who would have seen this as divine intervention, meaning that if God was helping them win then clearly they couldn't have been wrong or heretical in their beliefs. Once Islam had conquered they were also clever enough to consolidate and hold their power and they did this by leaving the conquered people alone. For example, old officials were allowed to keep their positions, Arab tradesmen were forbidden to take property from outside Arabia, there were no forced conversions to Islam and the Arabs even learned their admin from the conquered people, so the system remained efficient and understandable to all, in fact, it took 50 years for Arabic to become the language of many of these conquered places. Therefore, this lack of force or this very slow and gentle method of take over meant that not only could the Arabs take land, but they were also able to keep it as well as there were no uprisings or discrepancies with a completely new regime.