The answer depends on your own personal reasons. Some people are poly first, and find multiple relationships as a consequence. Others happen to find multiple relationships, and so realize they can be poly.
Me, I'm poly first. It took me years of not looking to actually find an additional relationship that was worth my time and effort.
So my answer is "Because I can't stand to cut off my options for no good reason."
Gralson: my husband (works out of town).
Auto: my girlfriend (lives with her husband Zoffee).
The most dangerous phrase in the English language is "we've always done it this way."