Here's why:
* It describes a noun: We use "credible" to describe something or someone, like a "credible source" or a "credible witness."
* It answers the question "what kind?": For example, "Is that information credible?"
However, "credible" can sometimes be used as a noun, specifically in the context of "giving someone credibility":
* "His expert testimony lent him credibility."
* "The company is losing credibility due to recent scandals."
So, while "credible" is most commonly an adjective, it can be a noun in specific situations.