Popularity, entertainment, media, ability to generate additional revenue, TV coverage. sponsorship? Any of these the word you're looking for?
About as warm as you can get without saying the word "demand". Simply put, there's more demand for men's football than there is women's football, so the men get more. Translate that to tennis and there's more demand for men's tennis than women's tennis. However, there's not that much of a gap between the two in comparison to football, so the difference is much less.
Perhaps if women played five setters demand for women's tennis would go up. Truth is though that it's not definite what level of demand women's tennis would reach if it did resort to five set matches. Who knows, demand might even drop.
As for the guys thinking that length of time played and size of fields matter, if we go by that then surely cricket would be one of the highest paying sports ever.