Because those are two different things. Polygamy is multiple marriage. All of the partners serve a similar function or have a similar role (that of spouse). And overwhelmingly, it means one man with multiple wives, where he is the “head” of the family and the women have largely equal subservient roles.
Polyamory is about multiple intimate relationships of various kinds. One of my partners is legally married to someone else. One I only see about once a month. They are very different kinds of relationships and meet different needs in my life. The man we see is also involved with my husband. In other words, it’s very, very different from polygamy.