Carried mycobacterium tuberculosis.
Eating them meant death.
Check out the Kosher Inspection, lookiong for the tubercles.
They had no knowledge of microbiology in ancient times, when the food laws arose. ANY animal has to be inspected (irrespective of keeping kosher, or halal) for signs of illness. Also, besides the low rate of TB in pigs (.4%) there's no transmission to humans from pigs. * It's a good guess that prehistoric hunting skills improved in order to nick the healthiest member of the herd of whatever game was being hunted.