Why does Herot remain empty for twelve years?

Herot, Hrothgar's mead hall, is not a happy place at the beginning of Beowulf. In line 148, we hear that for twelve winters, "twelf wintra," it effectively stood empty because of the plague inflicted upon it by Grendel, whom the text describes as a "wergan gastes," or a wicked ghoul. "Gastes" is an interesting term here because it gives rise to the modern English word "guest," but is also the ancestor of "ghost"—Grendel is not a wanted guest in this place. Indeed, Grendel has been perpetuating terrible slaughter for many nights upon anyone who dares to sleep in the hall, with the result that anyone with any sense (and not wishing to be killed) chooses to sleep elsewhere. Eventually, everybody stops spending the night in Herot at all, in fear of what might be done to them by Grendel.