The problem with polytheism is very clear, what god from the pantheon would the people worship ?
In Ancient Greece for example, Zeus wasn't the top worshipped for most of the time overall. It was taken by Athena and Apollo. Despite Zeus being the "king of the gods" in Ancient Greece.
Not to mention other cities took on patron deities they revered more than others, so polytheism can divide rather than unite.
Monotheism on the other hand, is much more inclusive by havingn a single god to worship. Especially when its a universal religion like Christianity and Islam. Different sects still emerge, but its nowhere near as divisive as polytheism. Having more grounded scripture is also a good point in favor of monotheism.
Thats the main reason why monotheism exists today while paganism-in general- is nearly extinct. And since followers of Abrahamic religions exist, AND they believe their scripture to be holy and correct. Its more than enough reason to believe that paganism is false in their eyes.