Here's another reason. Basically the mainframe software that runs a lot of the dispatch/gate processes is pre-historic. So they'd have to spend a lot of money re-coding it to actually generate the Postscript output that a modern laser printer needs to render pages.

"the printer" isn't just for the passenger manifest, but there's lots of other business processes, maintenance documents/releases, etc., that are printed from it on a rather time sensitive basis. With the sort of "change management" processes that are in place at a modern full-service airline, it would be an enormous undertaking to re-do literally everything that has been developed the years on the 'legacy' systems, train everyone, have a roll-out, etc. Its just cheaper to buy dot matrix printers and carry on the existing systems. Airline IT tends to move at a glacial pace, and most customer service graphical "terminals" are just front-ends, fancy or often not-so-fancy, for old mainframe applications.

Also, believe it or not, there's still outstations in which not much connectivity is available. An entire airline check-in and boarding/dispatch documentation generation operation for an outstation with the terminal emulated systems and CRT-attached printers, can probably run 5-10 check-in stations on a 9600 baud modem. Not that expensive of dial up circuit through an international carrier for the few hours a day it might be needed. But if you have the modern fully graphical stuff, that requires a lot more capacity that may very well only be used once a day.