Gendel is unable to attack King Hrothgar's throne because Hrothgar is protected by God. Since Grendel is a descendant of Cain, a reference to the Biblical Cain, he ultimately fears the repercussions of the sins committed by his ancestors. Grendel knows that his power is not greater than God's, and although Gendel is drawn to the mead hall to feed, he has respect for the powers that he can not defeat.
Where did Grendel’s parents come from?
What do the warriors do about Grendel’s attacks?
I don't know if the idea that because Hrothgar is king that he is protected by god. From my understanding Hrothgar and his people still believe in a more paganistic deity, so I don't know that the "God" that Beowulf comes to rely on throughout the story is really protecting him because he is king. I know this doesn't answer the original question but I wonder the same thing and that answer just doesn't sit well with me.
Hrothgar is king, and thereby he is protected by God. Grendel fears God because God exiled his ancestor, and grendel fears that, that might happen to him.