Because emotion immediately adds a layer of complexity.
Poly on paper is as easy as a regular relationship. Once you throw in emotional responses logic for most people gets tossed out the window.
This idea of everyones relationship being unique.. In the end that's peoples need to want to feel like they are part of a group, but being left to justify any failures to it's uniqueness. It's a simple copout. Its better than blaming a tough relationship on the people involved.