I've got to think most of the reasoning has to be because of interfacing with software that in general is really old. Dot matrix printers in general you could pretty much just dump the characters that you wanted printed out the parallel port and the printer would produce that output. No drivers or anything. In general, lasers and ink jets require more processing, and you're not just sending raw characters to the printer. Course, in general they're more capable of printing graphics too because of that, but that's not likely a need for the airlines.

Being able to load up a whole box of paper instead of unwrapping reams probably doesn't hurt either.

I'm assuming these are still in general parallel port driven, which means the parallel port continues to live well past when the manufacturers started to try killing it off.