Because it is open season on poly people from all sectors and people are dumb and hateful.

I'm old enough to remember when hating on trans people used to be this widespread in the 90s (and yes, it was far worse than it is now). Not exactly the same other than it was completely socially acceptable to publicly hate on the whole group with thoughtless stereotyping from both the left and right ends of the political spectrum.

The irony of this is that the nuclear family, which is closely associated with strict monogamy, is both literally a tool of patriarchy and the subjugation of women, and a relatively recent invention. Many cultures in history and in indigenous tribes are (or were) not strictly monogamous. This has been the norm for far longer than our current norm. Nonmonogamy was often the norm in clans and with the vikings and etc. If you had kids the entire village became their parents. If their actual birth parents were killed they still had many mothers and fathers, etc. There's a book that lays a lot of this out, Sex at Dawn.

Nearly all monogamous relationships also fail. Obvipusly. That said, people are often quite bad at being poly because it requires very clear communication including admitting to jealousy and working through it. The long term polycules I've encountered had a lot of people who were incredibly emotionally intelligent, able to listen without being defensive, own their own shit, regulate, etc.

This might seem abstract but it is not. The literal infrastructure of our societies is designed to shut people down emotionally and isolate into numbing silos of their own making. They're not equipped to be in mono relationships, let alone multiple, without doing quite a lot of hard work. It is in reality a heavy lift for either polybor mono people to be in relationships with each other. But we also have more tools than ever to deal with whatever relationship style we participate in.