Why? Marriage was around a long long time before Christianity or even Judaism. There were hue portions of the world which that framework didn't touch for centuries after it's conception. Even if we were to restrict our view to those polygamy still dominates large parts of their history - which is far from the view of "traditional" marriage.
https://en.wikipedia.org/wiki/Marriage#History_of_marriage is well cited, and links to other, more in depth articles concerning Greek History, Chinese History and others where "traditional" marriage was far from the norm (and around for much, much longer).