You know how people fight over things that happened hundreds of years ago? Its like that. When people who have been here for 100+ years hear about "family history" it often involves stories of say the potato famine in Ireland. While intellectually they understand Ireland is no longer like that (indeed they may even have visited it to get in touch with there roots). Emotionally they have a lasting impression of a America being the promised land. Their ancestors fled elsewhere to come here, thus here must be the best.

The main and important exception to this is of course African American's.