From what I gather, the most common, although very vague explanation in anthropology is that polytheism is the natural evolutionary stage of religious tradition after animism. read more
The tradition of monotheism developed from polytheism, just as polytheism evolved from animism. Judaism arose likely from the Canaanite faiths of the Levant, the same brand of polytheism that would give the world the Carthaginian cult of Baal Hammon and Melqart, the king of Tyre. read more
Same can be argued about Muhammad, who also brought word of monotheism across to even more of humankind. The Qu'ran talks about how the society Muhammad grew up was drowning in vice, idol worship, and sin, and the rise of Islam was something of a 'cleansing liberation' to create a pious new society. Almost like a religious revolution. read more
TL;DR: Hebrews believed in many gods; with time, Yahweh evolved by assimilating other gods and differentiating himself from them; In the 7th century, King Josiah made it so that they would worship only Yahweh; monotheism was a consequence of the Babylonian captivity, and Christianity did the rest. read more