Wild guess, pork was traditionally a fatter meat, most sausage has a lot of fat.. Also pork intestines where used for the cases, so butcher a pig and use his intestines for the casings. Somethings change some don't
Sounds good to me. If the meat is naturally somewhat fatty feeding and/or breeding over many centuries would develop the desired product. Now both tradition and price will allow it to maintain its position as the prefered sausage meat.