How did they manage to stay in Heorot even if Grendel is attacking there every night for 12 years?

Hrothgar was the King of the Dane, thus, the ruler over many, many people. Heorot was the center of his kingdom, to have left would have been to acknowledge defeat. No king would have done this at risk of being judged lacking. This is, of course, my opinion. We are not given these details in the text.