It seems like whenever I read about an ancient civilization, they mostly have a pantheon of gods and godesses. Today though, it seems like virtually all major religions, aren't polytheistic (Christianity, Judaism, Islam, Sikhism, Bahai, Druze etc are all monotheistic, and while I don't know a lot about Hinduism or Buddhism, I've been told that they aren't polytheistic, even if they aren't quite monotheistic either). Why were the polytheists so dominant in the past, and what changed to make them almost gone now?
Monotheism did exist in the ancient world. The main examples would not only be ancient Judaism, but Atenism - the sun worshipping religion of the Egyptian Pharaoh Akhenaten, and Zoroastrianism, the religion of the ancient Persians. Zoroastrianism worshipped one god - Ahura Mazda, and belief was very widespread in Iran up until the Islamic Conquests. However it’s certainly true that monotheism came to dominate from the Middle Ages onwards.
Polytheistic belief was, in ancient times, generally quite lucid, in comparison to the more strict beliefs of Christianity or Islam that have a single creed and are more united. Polytheists didn't consider themselves a follower of a particular religion - the gods simply existed. People living in Greece, for example, wouldn't deny the existence of Marduk or Ra, but simply identify these gods with one of their own. Polytheist pantheons weren’t exclusive. People may worship a particular god or belong to a specific cult but (with the exception of Judaism and Atenism), they wouldn’t consider themselves a follower of an exclusive faith. Polytheism and animism would have continued to be the dominant form of religious belief it weren’t for the success of monotheism.
The prevalence of monotheist religions today is largely due to the spread of Christianity. This influenced Islam, which in term influenced Druze and Sikhism (to a degree). When Christianity became the official religion of the Roman Empire, it spread rapidly amongst the Romans and the Barbarian conquerors who succeeded them. Because Christians and Muslims believed polytheism to be ‘false’, they promoted conversion rigorously through missionary work and conquest. By the time of the Muslim conquests, most of the Middle East was already Christian or Zoroastrian. Furthermore, religion became more of a political tool and so paganism, in Europe in particular, shrunk at the Abrahamic religions’ expense. Monotheist religions tend to have a more unified system of belief, in that people all believe in the same god, adhere to the same creed and philosophy. It thus inspires a greater solidarity amongst its followers, and sparked common identity.