This is a good historical perspective. In modern times, IMO, this is driven either by tradition, religion, or simple jealously. It's harder to control someone into a situation where you can restrict their contact with other potential mates, if their partner is "the jealous type".

Which isn't to mention that until very recently, compared to the whole of human history, women were subservient to the men in their lives. Often as property. Owned by their father until marriage, then owned by their husbands. This would have factored in, and especially with religion reinforcing the notion of the man as the head of the household, and promoting monogamy, a lot were living that life.

Polyamory and polygamy while fairly old ideas, are still fairly taboo for most, even if they would otherwise be open to it. There have been some that accomplish it in the modern era, usually in small clusters of people, but finding near-by, like-minded people is usually difficult at best, and impossible at worst.

There's also the fear of disease, since we started to identify sexually transmitted infections, which has only really been happening in the last ~100 years, give or take, there's an aspect of concern when going from a single partner to multiple. Obviously there are protections that exist but nothing is absolute. Some may limit their options simply so they do not have to worry about whether their partner(s) have picked up anything from others, which could be passed to them.

I'm sure there's a lot more that can be considered for modern monogamy, but I think those are the big ones. Everyone's choices are their own with their own personal driving forces to the decision. I think at a basic level we're typically wired for it, based on hillock's assessment, we've evolved to want monogamy for the benefit of our offspring; I think that's changing, since we don't bed wrestle for the purpose of making more humans most of the time anymore; even still, that biological drive to stick around to support the child bearing members of the species is still very strong even if there's no children, and no expectation of children involved in the process.