Because misdemeanor domestic battery convictions under current law preclude people from buying guns.
Here’s the list from the ATF: If states have knowledge of any of these, it should be turned into NICS.
Are there persons who cannot legally receive or possess firearms and/or ammunition?
Yes, a person who —
(1) Has been convicted in any court of a crime punishable by imprisonment for a term exceeding 1 year;
(2) Is a fugitive from justice;
(3) Is an unlawful user of or addicted to any controlled substance;
(4) Has been adjudicated as a mental defective or has been committed to a mental institution;
(5) Is an alien illegally or unlawfully in the United States or an alien admitted to the United States under a nonimmigrant visa;
(6) Has been discharged from the Armed Forces under dishonorable conditions;
(7) Having been a citizen of the United States, has renounced his or her citizenship;
(8) Is subject to a court order that restrains the person from harassing, stalking, or threatening an intimate partner or child of such intimate partner issued after a hearing
at which notice was given to the person and at which the person had an opportunity to participate, and includes a finding that the person subject to the order represents a credible threat to the intimate partner or child or the intimate partner OR explicitly prohibits the use, attempted use, or threatened use of force against the partner; or
(9) Has been convicted of a misdemeanor crime of domestic violence cannot lawfully receive, possess, ship, or transport a firearm or ammunition,is prohibited from shipping, transporting, possessing, or receiving firearms and ammunition.
A person who is under indictment or information for a crime punishable by imprisonment for a term exceeding 1 year cannot lawfully ship, transport, or receive a firearm or ammunition. Such persons may continue to lawfully possess firearms and ammunition obtained prior to the indictment or information, but cannot do so once the conviction becomes final.
[18 U.S.C. 922(g) and (n); 27 CFR 478.32]