It's a fact 80% or so of criminals committing crimes get their guns through associates, straw purchasers, or theft. They will never register those guns.

Another portion of owners buy guns in private sales but will never register them, since prohibited possessors are protected from registration laws.

Another portion will dispose of the gun after a crime, because it's worse being caught with a murder/crime weapon than failing to report it stolen/lost.

The small percentage of crimes that have a gun recovered (at the scene or not) which have serial numbers (recovered or not defaced) can still be traced to the original buyer through other means. 4473 forms are kept by dealers. Dealers no longer in business turn their records over the the FBI.

The small number of crime investigations that might get some small benefit from tracing a gun to an owner isn't justified by the cost of tracking hundreds of thousands of transactions annually.  Nor does that justify the impact to lawful owners who have to take their time and spend their money complying with registrations laws.

COST > BENEFIT