People may have told you that only indefinite nouns take the final -n sound, but that is incorrect. Many definite nouns take them as well. It's the definite article that prevents the final -n from sticking to the word, but if a word is definite because it is a proper noun for example, then many proper nouns do take nunation. Feminine proper nouns and most proper nouns of foreign origin don't, however.
Because it's the subject (مُبتدأ) of a nominal sentence (جملة اسمية). And the Nominative Case (حالة الرفع) means that the noun is marked by a Dammah (or an equivalent) on the last letter.