Unfortunately this is true. We are not just blaming woman for it. But to understand it we have to know the psyche of a women.

The one reason I can think of is in US girls suddenly  find a new kind of freedom which completely change them. If you look at Nepal girls are  usually very cared and have restricted freedom in Nepal compared to boys. So the effect of this freedom in US is more on Girls than boys.

Secondly, it is difficult to survive in US. So girls are constantly on a look out of guys with some money which will give them some sort of security security. My friends girlfriend left him purely because the new guy have more money.