Why are Hrothgar and his warriors powerless to stop Grendel ?

Hrothgar's warriors have been unsuccessful in killing grendel, and as a result, the best of his warriors have died. None of the warriors matched grendel in strength.

So Grendel prevailed and fought against right,/ one against all, until empty stood/the best of houses. That time was long; twelve winters' length the torment endured. . . .