Why do asian families cut ties with children because of divorce?

I was married to a Vietnamese woman for 12 years. We have two boys. Do to some major issues that she has (apparently her family to), I was awarded full custody.

We moved to the same town that most of her family lives in. The whole family cut my kids out of their lives. Regardless of what happened between their mother and me, why did/do they have to suffer the loss of an entire family?

It's been 5yrs and it still bothers them a lot!

How can they do this?