In this question, we are clearly not talking about supply and demand in the technical economic sense. Instead, “supply” would refer to the number of women who are willing and able to run for office in a credible way. “Demand” would refer to the number of voters who want to vote for a woman. Together, these factors do affect how many women will be elected to a country’s parliament.
In order for women to be elected to a parliament, women have to run for office. This is not something that will necessarily happen. It may be that women are not attracted to politics. It may be that there are not enough women at this point in a country’s history who have the experience and background that are needed to make a credible run for office. There can be other reasons. The point is that the number of women who actually get elected is influenced by the number who can and want to make a credible run for election.
In order for women to be elected, voters must vote for them. If voters do not care whether women are in parliament, they will be less likely to vote for women. Furthermore, if they are actively opposed to having women in parliament, they will be even less likely to vote for women. The degree to which voters want women in parliament will help to determine how many get elected.
Thus, “supply and demand” can help to determine the percentage of women in a country’s parliament. They help to determine how many women run for office and how many are elected.