Many more men compete in the marathon than women and so more men have come along with the talent to lower the marathon WR and have dedicated their lives to doing so.
Interesting point. I found http://www.runningusa.org/index.cfm?fuseaction=news.details&ArticleId=332
In 2013, 43% of the marathon runners in the US were female, thus 57/43 = 1.3 times more men; and in 2000, it was 38%, so 62/38 = 1.6 times more men.
So you are correct to state that "more men have come along...", but it's not even twice as many.
Maybe all we need for a sub-2 is a talented man to come along, and use ostrich blood and MÃ¼ller-Wohlfahrt's expertise.