There is a huge gender gap and the men discriminate against women and prefer to play other men. Thus the women get worse/less diverse practice than men which leads to them being worse at chess on average than men.
This leads to them not qualifying for tournaments, which means even less chances to practice because the best way to get better are tournaments. Thus the womens league so they can at least play their equal in a tournament setting. Also having a lower entry barrier for women hopefully motivates more women/girls to play chess.