Detailed flight info from the BTS is limited to major air carriers (as defined by the government). You'll notice that Comair and ExpressJet currently meet this definition even though they are regional carriers for the big boys, but in years past that was not always the case (e.g., 1997 Comair info is missing).

Unfortunately for me, Midwest Airlines has never managed to crack this "major airline" categorization, so I have many holes in my flight data. Maybe that will change in the future with their merger with Frontier and Republic.