How did monotheism become so popular?
The best way to begin answering this, is with an analogy. Would science be better off having endless theories for every subject, such as ten versions for just evolution? Or, would it be better to reduce all that variety to best of the best? The first would be like scientific polytheism and the second would be scientific monotheism. Based on discussions in these forums, the theory of evolution does not like strange gods before it; Creation or even rational evolution without dice and cards. It prefers the monotheism of the status quo. Science is not about Democracy, but the final truths of nature, that exist beyond endless free speech opinions.
Unlike modern science, that can be reduced/condensed based on experimental results, it was not as easy to do experiments with the gods. However, common sense tells everyone, that simplifying and getting everyone on the same page, was more efficient than anything goes.
Polytheism is also very similar to the modern approach of specialization, where people become experts in a small piece of the much larger pond. Back in the day, there would be a god or goddess for water, wine, war, love, death, etc. People would worship the god or goddess that best reflected their special skills and needs. Like a specialist you may know of love but not of war. Monotheism was more like a complete generalist education, where you knew something of war and love.
The main problem with the specialty approach, then and now, specialities do not interface easily. This leaves gaps in the bigger picture at the borders between specialties. We don't often see biology merged with physics. With specialty, one can lose track of the forest; the biggest picture of all science, because of all the specialty trees. Often this includes specialty jargon; how to worship. Monotheism sort of offers a view of that same forest, from the top of the adjacent hill, allowing you to see all things as merged. You may not see the details, but from there, you can see how the parts of the forest, mesh on the largest scale. Merging biology and physics would be easier from the hill; Generalist.
It also had to do with consciousness. Polytheism would be sort of be like having a multiple personality disorder, where separate personalities or even persona (social masks) appear with each formal aspect of social interaction. That mask behavior will cause your personality to dissociate. It was better to have all these parts under one leader; be yourself, so you can laugh, cry, sing and yell and still be the same person, that your family, friends or business associates know.
Monotheism is like a theory of everything in the sense of one creator to wear all the hats, that were once filled by a wide range of specialists. This is also connected; projection of the firmware, behind the human personality. Polytheism was more about the firmware, that is in the juke box of instinctive expressions. While polytheism brought this to the one source; inner self. The inner self wears all these masks. Monotheism, was part conscious evolution; more access to the core of the brain and inner self.