If we look at the evolution of religious thought we see a strong correlation between the religious beliefs of a group and that groups base of knowledge about the world and the groups size and social complexity.
In the smallest, pre-metal, hunter/gatherer societies we see animism as a way for these early peoples to explain that which is unknown. As peoples understanding of the world begins to become more sophisticated, ideas on the supernatural must adapt. Additionally, the rules and customs that are required for human beings to live together in some order begin to be integrated into developing religious myth systems. As small bands grow into multi-band tribal communities, which grow into city states, which grow into empires, religious beliefs and customs had to evolve and adapt to accommodate increasing complex social structures.
So as society increases in size and complexity, we see the evolution from animism to shamanism, to polytheism, and on to anthropomorphic monotheism and more abstract ideas of a monotheistic entity. I think it can be argued the early ideas of monotheism were greatly influenced by the Hellenistic Age of Greek Philosophy.
I would also say that later advocates for monotheism recognized that diverse beliefs in many different gods reinforced division and sectarianism. Advocating monotheism is a way to get everyone under the same religious umbrella, using the same associated morals, rules, and social customs, thereby creating a unified and uniform identity.
I think this is exampled in the expansion of Christianity with the Emperor Constantine and his conversion of the Roman Empire, and also with Islam, were Muhammad campaigned and converted the Arabian peninsula from a dominant polytheism to a monotheism that Muhammad adapted to fit with culture of the Arabian peninsula.