Any ideas as to why they may have been denied? Could it be the DOJ is hiding something that they don't want AA/US to see? If everything is being done the same, no reason to hold any previous info back, right? Nah, none of our fine government agencies would ever do that.
The lawyers for AA and US essentially wanted to review the DOJ files on those prior mergers, which is understandable. Hiding something? Probably.
Let's assume that in the files of the NW-DL and UA-CO mergers are some memos written by some Antitrust Division staff lawyers analyzing the reduction in competition and concluding "this merger [NW-DL or UA-CO] is a clear-cut violation of Section 7 and the DOJ should file suit to enjoin it." And, of course, those mergers were not enjoined.
While that would make for some fun and interesting reading, such a memo has no relevance to this case. Whether this merger violates the law depends on the reduction of competition resulting from this merger. The failure of prior, incompetent, Attorneys General to sue to block those prior mergers doesn't affect the legality of this one.
There is no legal doctrine that says "the government's failure to protect its citizens from antitrust violations in the past means that it must stand down in future mergers so that all participants in that industry have a chance to harm consumers and extract higher prices from reduced competition."
If there was legal precedent that established such a doctrine, Chip would be cutting and pasting the entire ruling day and night, posting it here and everywhere else. The union talking points would contain the citation to that precedent. Laura Glading would be citing the case nonstop.