Muslim armies did not force their religion on Jews and Christians after their initial conquests primarily because these two groups were considered "People of the Book" (Ahl al-Kitab) in Islamic tradition. This status recognized that Jews and Christians had their own scriptures and shared a belief in one God. As a result, they were generally granted a level of religious tolerance in exchange for paying a tax (the jizya) and acknowledging the authority of the Islamic state.
The rationale was rooted in the desire to maintain social stability and order in newly conquered territories, as well as the theological belief that people should not be coerced into faith. The concept of religious tolerance allowed for the coexistence of different faiths within the Islamic empire, creating a more diverse and pluralistic society. Thus, it was not simply fear of divine retribution or other factors listed in your options, but rather an acknowledgment of these shared beliefs and a practical approach to governance that influenced the treatment of Jews and Christians.