It is all about relationships
Monogamous morality is very recent in human history; for thousands of years before that, it was far from being the normal lifestyle because, well, it is not normal for humans to have one lifelong exclusive sexual relationship anymore than it is normal to be limited to only one friend or one child. Humans are social animals, we strive for relationships.
So, why use poly instead of polyamorous, non-monogamous or open relationships, or...? Because poly simply defines multiplicity (from Greek: much or many) without any other implication. And it is short and simple!