Here's why:
* Adjectives describe nouns. "Homesick" describes a state of being, like feeling sad or longing for home.
* Common nouns are general names for people, places, or things. "Homesickness" is a common noun, but "homesick" itself is not.
For example:
* "The homesick child missed her family." (Homesick describes the child)
* "Homesickness is a common feeling when you're away from home." (Homesickness is the noun being described)