Most academic research into polyamory has found that the community is overwhelmingly white, educated and affluent. The academics have also speculated that people from other demographics likely face more social backlash for deviating from social norms. They fear ostracism from careers, church, and rigid social expectations.
Anyways, it financially makes sense to have more than 2 working adults in a home. The West's nuclear family is a very new trend, and not a very successful one IMHO.