Why is Grendel a flat character in Beowulf?

That is because there is no specific character development or purpose or use of the character. His purpose and use are to be the evil that would get beaten by the main character, in this case, Beowulf.

Explanation:

Flat and static characters square measure somewhat necessary in texts. whereas their lack of description provides readers with very little to no info regarding the character, several are stock characters that readers straightaway acknowledge (even with the restricted description).

In regards to the character of Grendel in fictitious character, one might create terribly completely different arguments concerning him being a flat character as a result of the author offers restricted background info on him

That is because there is no specific character development or purpose or use of the character. His purpose and use is to be the evil that would get beaten by the main character, in this case Beowulf.