Here's why:
* Common words: We need common, everyday words to form a sentence. Most common words don't start with "n".
* Nouns: We need nouns (people, places, things) to make a sentence. Many nouns don't start with "n".
* Verbs: We need verbs (action words) to make a sentence. Most verbs don't start with "n".
Fun Challenge: You could create a sentence using made-up words that all start with "n", but that wouldn't be a real sentence in the English language!