I'm not poly, but if I had to guess, the resurgence of that is the pendulum swinging back the other way against how atomized social media has made us. Many people, myself included, don't many or any friends. Poly is one way of closing that gap for those who are open minded and searching for deep familial like bonds.

The coming advent of genetic engineering will make poly more interesting as it be possible for more than 2 people to be biological parents. In fact, we see this already in popular sci fi like The Expanse.