The 1986 registry cutoff was part of the Firearm Owners Protection Act. The Act was intended to protect firearm owners from unconstitutional practices (including prosecution) by the ATF. The Act also was intended to keep the government from keeping a registry of non-NFA firearm owners.
The Hughes amendment to the FOPA closed the NFA registry. As a result of limited supply, prices on transferrable machine guns skyrocketed. Gun control through economics.