I'd just toss it back to them -- why not?
But, if you want something a little different, perhaps "Because it suits my personality and desires, and has been a good fit for me and my partners. I know it's not for everyone, but I appreciate the honesty and freedom."
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.