Of course Monday final doesn't attract viewers to begin with. Monday is not a favourite day for many people especially for those who have day jobs. I mean who wants to watch a men's final on a Monday at 5pm? So the rating has its own downfall written all over it to begin with. Also, tennis is not all that popular in the US. I mean why was there a lower rating for the women's final given that it was played on Sunday?
Things are changing however next year when the finals will be played on Sunday and semis on Friday. And the first round matches will be played over two days instead of three. Not sure when they are getting the roof on the main show court but that only helps top players and I am not a big fan of that.