Because the radical patriotism that nationalism promotes is the blind idea that your country is good and can do no wrong, thus any attempt to dramatically change it would be an utter insult. If people from a completely different culture enters the country, a nationalist will think that the person wishes the country's systems to change. (Or at least that what i think nationalism entails, though don't attack me for thinking so.)
What do you think about the results of the UK brexit?