Why is the head called Lord of the Flies?

In the literal sense, the head is called Lord of the Flies because flies did, indeed, scavenge the rotting pig head, giving Golding the ability to refer to the pig head as the Lord of the Flies.

"Lord of the Flies" is directly translated to "Beelzebub" - a deity sometimes even referred to Satan himself. The appearance of this pig signifies the big turning point in the novel as the boys become increasingly savage afterwards.

When Simon encounters the pig and has a hallucination, he comes to realization that the beast is not on the island - it is in all of us.