Here's why:
* Adjectives describe nouns: "Important" describes the quality or characteristic of something. For example, "This is an important meeting." "Important" tells us about the meeting.
* Nouns are things: Nouns are words that name people, places, things, or ideas.
To use "important" as a noun, you would need to use a related word like "importance".
For example:
* Adjective: "This is an important meeting."
* Noun: "The importance of this meeting cannot be overstated."