Here's why:
* It names a thing: The alphabet is a set of letters used in a writing system.
* It can be the subject of a sentence: "The alphabet is a fundamental part of language."
* It can be the object of a verb: "I learned the alphabet in kindergarten."
However, there are instances where "alphabet" could be used as an adjective if it's describing something:
* "Alphabet soup" - This phrase uses "alphabet" to describe the type of soup.
Generally, though, "alphabet" is considered a noun.