Probably something to do with the old saying,"The only thing that's wasted on a pig is its squeal".
Meaning that just about everything from a pigs carcass is used for human consumption,nothing left for the animals.
A lot of pork products contain salt which is bad for dogs.
An abbattoir near me set up a pet food processing plant to make a pork based dog food. They called it McMillans and it was trialled by people in the area, but it was soon taken off the market because it was making the dogs ill and another side effect was a build up of cholestorel characterised by coloured patches in the dogs' eyes.
I thought it was to do with the fact that we, as humans are the closest thing that taste like pork and if they got too hungry, they'd be biting chunks out of us!!! LOL!