There are a couple of factors which come in to play here. Namely, cultural norms and financial stability.

In my experience, women from countries with higher poverty rates or where some sort of cultural stigma has been placed upon them due to such factors as skin color or social status will be considerably more aggressive in pursuing men who can give them an opportunity to lift themselves and their family out of a bad situation.

Probably the most high profile example of this situation is Melania Trump. As Trump's female escort on retainer, she has done much to improve the living situation for herself and her family back in Yugoslavia. For this, if nothing else, she deserves some sort of comendation.

In countries where women have a more level playing field, the need to be aggressive in the pursuit of a good mate is less pressing. Time availability plays a part as well. With factors such as increased responsibilities in the work force and higher wages, women have no need to relocate, through marriage, to places where the opportunity to receive an education and get a good job are readily available to them.

Cultural habits? For so long it's always been men that have had the freedom and responsibility to approach a woman.

Without the social pressure to make the first move there is less driving them beyond their nerves? That would be my guess.