You know why.
All of the legal gun dealers possess Federal Firearm Licenses. Part of their responsibility is to maintain a hard copy of all firearm transactions in their stores, i.e., BATFE Form 4473. These records are supposed to remain on the premises of the business. The BATFE has the authority to in come any time and inspect those records. Should the business close, BATFE takes possession of these records. The question that begs to be asked are these old records gathering dust in some federal warehouse or are they being secretly compiled into a firearm registry?
I, too, marvel at how LEOs were able to match the serial numbers from the recovered firearms with the Form 4473s or the NICBCS inquiries.
It is my understanding that the FBI's NICBCS is supposed to purge all digital records of background checks after a certain time (way less than a year). Sure would like to see someone on FR confirm this. We have been paranoid for some time (since 1998 when the NICBCS went fully operational) that the FBI makes backup copies of these background checks with the eventual illegal goal of developing a national firearm registry.
I believe I heard some talking newshead say that the mass murderer had purchased 30 of his firearms this year. That suggests to me that the FBI still had the NICBCS records and had not purged them.