I will post based on what Janaki has told in her first post.
In any marriage, marriage works when husband and wife develop love between them and understanding between them and as days go by become more and more caring, compassionate, gentle, tender and loving to each other. Also they are to be more and more with each other, present each other and available to each other. If it is polygamous marriage it becomes possible one wife gets to this position with the husband and in that case the other wives will be neglected. Dhrit in any case, even in monogamy could not give happiness to Gandhari because he was not at peace with himself and he had no love even for himself.
As far polygamous marriages, how did they happen? There could be only two possible causes. One is if a woman is prepared to marry a man who is already married. She could do so if he is a rich man or a king as that would settle her life instead of marrying a poor man who has no wife. In 'Abhijnana Shakuntalam', a dialog is such that Dushyanta tells, 'That person is rich. So he could have many wives.' It is also possible, she could marry an already married king because she was forced to do so, as if she doesn't marry him, the king could do havoc with her father's kingdom as the king is highly powerful. It is unfortunate no one asked the feelings of any woman who is forced to marry an already married person. Even Gandhari forced herself to marry Dhrit, because powerful Bhishma could do anything.