Before Christianity became a big concept, most countries seemed to have multiple gods. However after the Romans adopted Christianity as their religion, and imposed it in many more countries, a lot of monotheistic religions have developed. And now the biggest religions are monotheistic. What is the reason for this change in ideology?
Before we get to far into this answer I think it's worth noting that they two largest countries in the world by population India and China are dominated by non-monotheistic religions.
Regarding Christianity and Islam, I think you are focusing on the wrong thing it's not their Monotheism that has led Christianity and Islam to spread across the world but rather their commitment to proselytizing and expansion, as well as the success of the states who propagated their religion. It doesn't have much to do with ideology at all, and we can see by the third largest monotheistic religion Judaism from which both Christianity and Islam derive and which has not expanded to cover the world despite predating both Christianity and Islam. It's not then the characteristic of Monotheism that allowed these religions to spread but rather the characteristics of the religion themselves.
Christianity and Islam were spread both by states conquering lands and then promoting them as an official religion or through missionaries who often converted rulers through their lands these rulers who then promoted it as an official religion, and finally personal conversion. That said if you look at the borders of the European colonial empires and the Rashidun Caliphate you can get a very close match for the prevalence of Christianity and Islam. These states promoted their religion in the areas the conquered through incentives (whether active or passive) or force, or both combined. That is why Monotheism is so prevalent.