Here's why:
* Pronouns stand in for nouns. "Everything" refers to a group of things, like "all things".
* Adjectives modify nouns or pronouns, describing their qualities. "Everything" doesn't describe anything; it's a noun substitute.
Example:
* Everything is going well. (Everything is the subject of the sentence, standing in for a group of things.)
* The everything bagel was delicious. (Everything is used as a noun, not an adjective.)