The only thing that could prosper were the flies coming out of the Lord of the Flies.
At this point in the book, the boys have pretty much descended into chaos. The muggy weather confirms this. Nothing can “prosper” because they are stuck in a primordial state.
Nothing prospered but the flies who blackened their lord and made the spilt guts look like a heap of glistening coal. (ch 9)
The flies prefer the pig’s blood, because the pig head is symbolic of the Lord of the Flies. They prefer the pig to Simon. This demonstrates that the boys are intruders on the island, and the island is returning to the pre-civilization state it has always been in before they came.