It used to be ads; before the Internet there was limited advertising space that reached lots of people, so papers could fund themselves with their wide distribution even in the face of competition from TV. Of course, the Internet blew that wide open.

A couple of places have tried diversifying content towards what readers will pay for; NYT has done probably the best job with the branching out towards cooking, Wirecutter, etc. that people do want to pay for. Buzzfeed attempted to do the same for clickbait to fund real journalism but that fizzled out.