Gwen Kansen asks, "Why is polyamory so popular now?", but I could reframe the question as, "Why have numerous once-minority pursuits, beliefs, and interests spread?", and the answer is the same, "The Internet." That's a true but not completely helpful answer, and it's more specific to say that anyone with niche interests, unusual beliefs, or non-mainstream pursuits had a lot of trouble and friction finding one another before the ubiquitous Internet, and so niche beliefs stayed very niche.