Its the internet (duh) but not in the way you might think. Sure, the tech savvy and the newer generations are increasingly turning to the web for news and shunning newsprint, but the real giant killer is Craigslist!!
Classified advertising has been the solid foundation of newspaper revenue since forever. Its local, its personal, its extensive, and its searchable (its “classified”). But Craigslist does it better, no question. More analysis is in this piece from the New Yorker magazine, “Out of Print; The death and life of the American newspaper“