The data on number of licensed firearms dealers is available here. Understanding this pattern goes a long ways towards explaining the changes over time in the rate of sales through FFLs at gun shows (see also here). Part of the reason for the drop is explained here.
The Brady Handgun Violence Prevention Act of 1993, known as the “Brady Bill,” increased the fees for federal firearms licenses from $10 per year to $200 for the first 3 years, and $90 for each 3-year renewal period. . . . .
Additionally, in 2004, ATF implemented an in-person application inspection program. ATF industry operations investigators now conduct in-person application inspections with all new firearms business applicants. Through the inspection process, investigators verify the identity of the applicant, ensure the qualification of the person(s) intending to conduct business, verify the business premises, and review the record keeping and conduct of business requirements to assist the applicant in complying with the law and regulations.