Monotheism origins seem to be mainly from nomadic desert demographics. Why?

I think the word "nomadic" is the key to your answer. Eastern and pagan religions tended to be more isolated and provincial while the western Abrahamic religions focused on spreading the message.

Even given that, the monotheistic religions didn't really "expand drastically" until Christianity (which is not really monotheistic, but that's another story). Judaism was restricted largely to tribal groups and didn't really spread until it influenced the formation of Christianity and Islam, which came much later and whose expansions were fueled by powerful governmental forces that promulgated it, often at the tip of a sword.

Another factor was the nature of the religions themselves, with eastern religions focusing more on spirit and inner reflection and western religions focusing more on prophetic mandates and governmental/political systems that dictated rituals and traditions affecting everything from sexual behavior, to diet, to clothing, to gender roles, etc. etc.

Most faiths have one God. Jews, Christians, Muslims, Sikhs, Hindus.... most of the word in fact. Few are polytheistic.

Actually Egypt had the first Monotheistic belief.

Monotheism origins seem to be mainly from nomadic desert demographics. Why?

--- Actually, monotheism was developed by the Persians who lived in rather mountainous terrain.

Desert tribes like the Jews and Arabs started as polytheists and merely copied Persian ideas...most like because Persians (at the time) were the most powerful state in the region. Which probably lead other tribes to believe monotheism gave the Persian people some mystical advantage in war.

But El, El Shaddai, Yahweh, Baal, Asherah were all Jewish gods at one time. Then Baal and Asherah got banned, and the other three were merged into one god to create a monotheistic religion.