Because you suspect that the cause of death may have been a disease which is communicable to humans:

https://en.wikipedia.org/wiki/Erysipelothrix_rhusiopathiae

But the vet determined that it was not erysipelas

Even if it wasn't erysipelas, they didn't know what it was. Eating the pig could've been fine if they at least were able to tell what is it that it had and whether it was infectious or not.

Eating animals whose cause of death is unknown is usually not a sound decision.

The pig had died at some unknown time before. It was summertime. The residual heat of the internal organs would have quickly spoiled the meat; this is why hunters, even in the winter deer season, try to get the guts out and the skin off as soon as possible, in order to let the heat out before it ruins the meat. Also, the pig died for a reason. Could have been a twisted intestine, or it could have been some sort of disease, but in any case the vet couldn't figure out what was making the pig sick which means they don't know if it's communicable.

I grew up on a farm. One winter day, years back, my sister's sheep just started dying. 3 or 4 of them died over the course of an afternoon, for no reason we could discover, and then the rest of the sheep were fine. We didn't eat the sheep, even the ones we had literally watched expire; it's just not safe.