In 2000, the low-fare carriers were much smaller than they are now; legacies were over 75 percent of the market, if I remember right. JetBlue was just starting, and WN
were smaller. A lot of medium-size markets did not have a low-fare carrier--places like GSO
. A lot of elected officials had reasons to oppose the merger, and it never helps when there's political opposition. I for one wrote two newspaper columns opposing the transaction.
Finally, as Bobloblaw noted, by the time the review and fight had dragged into 2001, the merger no longer made sense. The economy was weakening, and the combination of two high-cost carriers no longer seemed to make sense. When the Bush DOJ ruled against UA
-US, it already was coming apart.
By 2009-2010, low-fare carriers were much larger and secondary markets had more LCC competition, most legacies had reduced costs some in bankruptcy, and the Bush and Obama administrations were open to the arguments the carriers made for cost-synergy benefits from merging.