The reason why beef lung is illegal in the US is because beef lungs and other animal lungs can accumulate contaminants, like dust, asbestos and pollen from the air that they breathe.
And during slaughter, gastrointestinal fluids can also enter the lungs and could result in the beef lung potentially containing harmful bacteria or even parasites which could make you sick.
And it's also difficult for inspectors to thoroughly examine the beef lungs for any signs of contamination or disease.
Some people safely do eat beef lung in some locations or even from slaughtering of their own animals although the USDA doesn't believe the beef lungs are safe once cooked.
The U.S. Department of Agriculture or USDA banned the sale of beef lung and other animal lungs for human consumption in 1971.
In many countries including the Untied States it's illegal to sell lung as a food product, because there's a risk that gastrointestinal juices might leak into the lung during the butchering process.
The risk is very small, but eating a contaminated lung can make you very sick, so a lot of countries have banned it.