The reasons aren't complex at all. The women that have left have been quite open about their reasons. It's part of the pathology of the west that their reasons aren't accepted.
They leave because they want husbands and children. For whatever reason, they do not see a future in the west for them being married women. This culture has no promise for them. It is full of men who refuse to consider a stable relationship and think that abortion is the way pregnancy ends.
This is not new. Brides have been traveling the world for husbands since there was traveling.
Yep, the reasons are simple. Women are promised husbands and families.
Why don't these women think they can marry in their home countries? Western world has destroyed the family or even the whole idea of the family. Tradition family. They see crowds of queers on the streets...and decide to have realy strange family instead