There was a time when the only outlet for advertising was newspapers, magazines, TV and radio. So, one advertising pie with 4 slices. Newspapers generally got the largest slice.
Newspaper subscriptions did not cover the cost of the paper it was printed on. That was all covered by advertising revenue. The more paid subscribers you had, the more money you could charge for advertising in your newspaper, the more money you had to spend on a newsroom with reporters, photographers, editors, etc. And the more money you had to spend on production and delivery expenses.
In the early 2000s, along came Craig's List. Craig's list made its money by charging low fees for Employment ads. They provided free Classified ads. That was the first big nail in the Newspaper coffin, as Classified Advertising was most newspapers' bread and butter.
Anyone remember the huge classified sections in your Sunday papers?
Anyhow, as Craig's List ate into newspapers' revenue stream, other forms of on-line advertising also popped up (literally, pop-up ads). Newspapers went digital in an attempt to reach the on-line audience, where they can only charge pennies for on-line ads VS dollars for printed ads, their revenues suffered even more, and there you go. Print subscribers left in droves for 'free' content on-line (which was usually scoured from Newspaper websites). As newspapers could not afford to print papers, or deliver them to former outskirt customers, their paid subscriber numbers dropped, and they had to charge less for printed advertising. The toilet started flushing on the expenses, and down the tubes they go, one by one.
Advertising is still a source of income for newspapers, but in recent years, for the first time in history, subscriber fees (both digital and print) started to be their largest source of income. And it's pennies VS what they used to bring in. In many cases it is no longer enough to support a robust newsroom and production costs... And they go out of business.