I would propose that it was because many of the pre- 19th century nation states were already broadly defined as being the home of a single ethnic group (Spain, Portugal, Norway, Denmark etc.) , but more particularly, ethnic nationalism was the only way to convince the populaces of present-day Italy and Germany that unification was a good plan. I am no expert on Italian unification, but I know that Otto von Bismarck (The Chancellor of Germany at the time) saw the necessity of building a ethnic German identity as a building block for German unification. This came to fruit in the Franco-Prussian War of 1870, which resulted in German Empire being proclaimed in the French palace of Versailles. Sources and Further Reading: https://history.state.gov/countries/issues/german-unification http://www.history.ucsb.edu/faculty/marcuse/classes/133p/papers/paperimages/MapEurope1815.jpg Italian Unification: http://www.sparknotes.com/history/european/1871/section3.rhtml