Why are nations more likely to have cultural boundaries within them then nation-states

A cultural boundary is a non-physical boundary that differentiates different ethnicities and cultures based on their race, language, cultural traditions and beliefs.

A Nation-state is a type of state that is conformed only by one ethnicity. This is why Nation-states are less likely to have cultural boundaries than Nations. There would not be a lot of cultural boundaries in a society that is conformed only by the same ethnicity. Citizens of a Nation-State share the same language, beliefs and traditions. While in a Nation it is easy to find citizens that belong to different ethnicities, therefore they have different religions, languages or cultural traditions.