The reason polygamy occurs relatively infrequently (even where it is legal) is that

Polygamy, even where it is permissible, inevitable leads to a micro-scale depletion of resources. Oftentimes, polygamous relationships inevitably lead to large numbers of offspring. So in addition to the wives, there is a large number of individuals dependent on a limited number of resources, this could be financial, emotional or physical resources, like food. Even without excessive numbers of children, the emotional toll on the women involved is frequently considered extremely abusive emotionally and inevitably unsustainable, either way.