Aside from the fact that such a small portion of the population is polyamorous, another major reason why polygamy isn't and is unlikely to become legal is cause it's a legal nightmare. Divorces can already be extremely lengthy and messy affairs with just disagreements between two people, that would grow exponentially worse as you add more people to the mix. Not to mention all the other different legal benefits and responsibilites that come with marriage that'd need to be adapted to cases with more people as well as countermeasures to people who'd exploit the system to get unfair advantages. It'd become very very terrible, legally speaking.