The show on Smithsonian, Alaska Air Crash Investigators explains that the report is compiled by the local investigator(s), then sent to HQ in Washington, DC for final review before it is released.  My guess is as good as yours: workload, federal red-tape, etc.  Aviation is also EXTREMELY litigation-heavy, so it behooves these folks to get it right, at least as much as they can given the circumstances.  Just my 2 cents...