It was instituted to prevent people from taking advantage of their wives.
It was intended to avoid potential infighting between rival wives, which may also lead to the transgression of a number of biblical violations.
Rabbi Gershom was concerned lest the husband be unable to provide properly for all his wives (especially during the difficult times of exile).
The ban is intended to avoid the inherent rivalry and hatred between rival wives.
There is a concern that a man may marry two wives in different locations, which may lead to forbidden relationships between offspring.
It would seem that polygamy is not, and never was, an ideal state. The mystical works are replete with references to husband and wife being two halves of one whole.
(Sources removed, and grammar edited. See there for original version.)
According to all of this, if polygamy isn't encouraged and is far from ideal, why was it ever permissible in the first place?