Good question. Mr. Kvarnstrom may be able to give you more details than I could on the matter.
In general, the way people consume information in the various media has changed. Companies and the government are advertising differently. If a digital advertisement can reach 50,000 people through Facebook, Google or Twitter, but a local radio station can reach only 20,000 people, and if the company has had the same advertising budget for the past 20 years, then logically the company will choose the media platform that helps it reach as many people as possible.
I can't say specifically whether the change occurred 5, 10 or 20 years ago. More and more people use digital platforms for information, news, television, radio, telephone, and so on. The change was gradual.