Here's why:
* Adjectives describe nouns: "Different" describes a noun, telling us what kind of noun it is. For example: "This is a different book." ("different" describes the noun "book").
* Nouns are people, places, things, or ideas: "Different" doesn't fit any of these categories.
If you want to use "different" as a noun, you could say:
* "There are many differences between the two." ("differences" is the noun form)