Here's why:
* Proper nouns are specific names of people, places, or things. They are always capitalized. Examples include: John, London, Amazon River.
* Common nouns are general names for people, places, or things. They are not capitalized unless they are at the beginning of a sentence.
"Grammar" is a general term that refers to the system of rules that govern the structure and use of language. It is not a specific name. Therefore, it is a common noun.