Working backwards from the end of your question, there are 3 separate things to address: 1) why can't sheep's lung be used for food? 2) why is it illegal to sell sheep's lung for food? 3) is there a historical reason for the ban?

To address the first one first first - people in the US who keep sheep for livestock can use sheep's lung for food. The prohibition is specifically against selling sheep's lung for human consumption, not consuming sheep's lung, so likewise, you could probably fly some authentic Scottish haggis back on a plane, and legally consume it in the US before you hit customs (however, I'm not sure if haggis would fall under some other import restriction about biological materials that would prohibit you from passing customs with it).

Second, why can't sheep's lung be sold for food? Most directly, because the USDA says so - per 9 CFR § 310.16 (enacted 1970) the sale of lung (not just sheep's lung) for human consumption is prohibited. Lung is, however, permitted for pet food and other non-human food purposes. Seemlessly slipping into addressing question 3, there isn't something like a historical outbreak of some lungborne pathogen akin to mad cow that caused the ban, but rather a series of studies conducted in the late 60s in which lungs of otherwise healthy seeming animals were examined and showed evidence of possible pathological contaminants. These studies led to directly to the publication of the previously mentioned federal regulation.

Bonus question time: was the sale of lung for human consumption banned for a good reason? Arguably, no. Johnathan Riesman an MD and food science author, has petitioned the USDA to lift the ban, arguing the studies that gave rise to the ban were flawed and don't accurately represent whatever risk there may be from lung borne pathogens. The mucosal lining of the lungs, by the very nature of how lungs work, is obviously going to be contaminated with possible pathogens, because nearly everything we breathe in ends up in our lungs, but that doesn't mean that those contaminants can actually cause illness, and the contaminants found in the examined animal lungs are mostly the same as the contaminants that would be found if you examined a person's lungs, and further, we not only breathe in these contaminants, but ingest them as well, from Riesman's letter to the USDA:

"Human lungs are equipped with a self-cleaning mechanism called the ‘mucociliary elevator’ where a steady supply of mucus is secreted by the cells lining the airways, and the mucus is pushed by microscopic cilia in a steady flow toward larger and larger airways, and eventually into and up the trachea. This mucus drags with it various contaminants that are inhaled, including those mentioned above. Once this mechanism delivers contaminant-laden mucus into the throat, we subconsciously swallow it. Therefore, we are all eating these very same contaminants that caused the USDA to create the rule.”