Here's why:
* Old English: The Old English word for "cat" was "catt." To create a word for a small cat, they added the suffix "-en" which was commonly used to make diminutives. This is how "kitten" came about.
* Other Languages: Many languages have similar diminutives for "cat," like "gatito" (Spanish), "gatinho" (Portuguese), and "chaton" (French).
So, it's not a specific reason why a small cat is called a kitten; it's simply a word that developed naturally in the language to describe a young cat.