Because in practice it always puts the collective above the individual. Being proud of your country is not nationalism. Nationalism is collectivizing your citizens and "asking" that they put the interest of the whole above their own needs for the good of the country..
Again, this would be an Americanized form, way to apply, "nationalism". In reality, the European-rooted "nationalism" really has no direct American application. That form is almost the opposite of what you describe. A European application puts the shared, natural culture below the structured, recently established collective. The flowers and fables below the flags and fences.
Currently in America, however, we have a culture and nation almost as a one. For example, what culture do Americans have without the history of our colonies, our constitution, the progress of western expansion as directly tied to the growth of our nation, etc., etc., etc.?? And compare this to Germany or France. Where the cultures as existing on those lands are much more deeply rooted that the associated forms of current governments and national structures, or even the previous ones, or the ones before that. For those people, there is very little or no connection between culture and nation. If you were to ask a modern Frenchman today, for example, particularly a non-Parisian, about French national identity, he may very likely answer with something like
living in the 3rd revolution, because that reply recognizes that it is the French culture which achieved it, and not any form of organized structure or rule.