Why can't Grendel approach the throne itself in Herot?

because he had no love for what the throne represented.

Explanation:

Grendel was a descendant of Cain and he could not approach Hrothgar's literal throne because he had no love for what it represented that is God's throne. Hrothgar is God's representative on earth hence God made Hrothgar's throne distasteful to Grendel and impossible for him to approach.