Here's why:
* Proper nouns are capitalized. "Muggle" is a specific term created by J.K. Rowling and designates a particular group within the Harry Potter universe, making it a proper noun.
* Consistency with established usage. In the Harry Potter books, "Muggle" is consistently capitalized, and it's important to maintain that consistency for clarity and respect for the established canon.
However, if you're using "Muggle" in a different context, like a metaphor or a general term for someone who doesn't know or understand something, then it would not be capitalized.
Ultimately, the capitalization depends on your intended meaning and the context in which the word is used.