If the now-defunct Eastern Airlines in the 80s carried more passengers than any other Airline at the time why did they go under?

Management, unions... airlines are not a good investment

Poor management maybe

Eastern over leveraged their planes ... the combination of rising interest rates, higher fuel costs and point2point routes doomed them.

Atlanta and DFW made other airlines more price competitive ... hubs ...

they were acquired and liquidated by a private equity - who moved all eastern airlines remaining assets to other holdings. it was only a matter of time before eastern airlines would run out of cash, and without anything to leverage for loans, go under.