If one group doesn’t one to marry members of the other group because they prefer to maintain their minhagim or because they want to know the family more closely or because they think that by staying within the same tradition will make it easier for the couple to be happy together, that’s all fine.
However, I think that in many cases the main issue is that one group looks at the other as second-class jews and therefore don’t want their children to marry members of the “lower” group.
Unfortunately, this division seems to be stronger in EY and NY.