Grendel is unable to attack Hrothgar's throne because he didn't have God on his side. Hrothgar has protection from God, as he is the king.

The speaker says that God opposes Grendel, so Grendel was unable to touch Hrothgar's throne. Perhaps this is why he was unable to kill the man himself, despite attacking every night for twelve years. While the poem doesn't directly explain why Grendle is opposed by God, it's likely because Cain was a progenitor of Grendel. Cain was the first person to commit murder in the Old Testament and was cursed by God. He is the ancestor of the monsters in Beowulf.

If Grendel had been favored by God as Beowulf and Hrothgar were, he may have been able to approach the throne and kill Hrothgar. Instead, he kept up his murderous campaign for years but never killed the man who built Heorot in the first place.

Why doesn't Grendel attack Hrothgar's throne in Beowulf?

I think you're referring to lines 168-169 of the poem. These lines are very difficult to interpret because the pronoun articles are not clear in the original Anglo-Saxon, and the language itself is a bit odd, as if the scribe who copied these lines was not entirely clear about their meaning himself (note that the version of the poem we have today was copied many centuries after the poem was first written). It's not really clear whether the "gift throne" which cannot be approached respectfully is Hrothgar's literal throne, or if God's throne is implied. It could even be argued that Hrothgar, rather than Grendel, is the "he" who cannot approach the throne, as this would be a perfectly valid interpretation of the language. In context, however, it's usually determined that the "he" who cannot approach is indeed Grendel.

Generally, it is assumed that Grendel could not approach the "gift throne," 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, and Grendel is a descendant of Cain, so God made Hrothgar's throne distasteful to Grendel and impossible for him to approach. However, this is a heavily Christian reading, which once again suggests awkward later changes to the text, which was originally written within a pagan context. An alternative reading would be that Grendel does not dare to approach Hrothgar's throne to pay his respects to him—"gretan"—as a loyal retainer would, because he knows he has behaved in a lawless fashion and is outside of the king's graces. So, although he ventures into the hall itself, he deliberately avoids Hrothgar himself.

A king is a position that is believed to bring that person closer to God.

Grendel is supposedly the spawn of Cain (from the Bible, Cain is the first man to commit murder by killing his brother). Therefore, in religious terms, Grendel will not dare touch the thrown, because, in turn, that is too close a step to God.

Beowulf is noteworthy as the oldest surviving major work composed in English, although if we were to attempt to read it in its original form, Old English, we would find it incomprehensible, because the language has changed so much since the poem was composed about 14 centuries ago.

In those days, England was still in a state of religious transition, moving from the old pagan mythologies of the Celts and Anglo-Saxons to Christianity, which was spreading in Europe in Asia. However, such major changes don’t occur instantly, so we see in Beowulf a confluence of Christianity and paganism.

There are several instances in Beowulf that demonstrate this evolution. At one point, the Danes, in their fear over Grendel’s murderous rampage revert to paganism:

And sometimes they sacrificed to the old stone gods,
Made heathen vows, hoping for Hell’s
Support, the Devil’s guidance in driving
Their affliction off.

We also see that Grendel himself has the paganistic power of sorcery, as he able to cast a spell to help him terrorize the Danes:

. . . for that sin-stained demon
Had bewitched all men’s weapons, laid spells
That blunted every mortal man’s blade.

Such a power implies that the people of this time were willing to accept, and in many cases probably believe, that certain individuals have the power to cast spells.

Finally, the story shows that God is able and willing to intervene miraculously, in some cases, to help his followers. In those days, and for quite a long time afterward, it was generally accepted that kings were appointed by God. Hrothgar, king of the Danes, has a throne that Grendel is unable to attack, because he was prevented from doing so by God:

Though he lived in Herot, when the night hid him, he never
Dared to touch king Hrothgar’s glorious
Throne, protected by God—God,
whose love Grendel could not know.

Hrothgar's throne is protected by God so Grendel cannot touch it because Grendel, a monster born of a pair of monsters who were decended from Cain, cannot know God's love.  The story is a mix of paganism and Christianity because the Christian faith was beginning to spread across the land but people hadn't fully relinquished their paganistic beliefs yet.  The Christian references are full of reverence and mystery because the faith was new to people.