The term was coined because the mainstream media is no longer as dominant as it had been since the mid 20th Century, when the rise of network television and the consolidation of local newspapers by large chains squeezed out locally owned, and sometimes conservative, news sources. While earlier newspaper chains like the Hearst and McCormick interests, were conservative, the second generation of these families were liberal or sold out to liberal interests like Gannett or the New York Times. The Big Three old TV networks were owned by liberals from the start, as was CNN under the leadership of Ted Turner.
What made the legacy media no longer mainstream were:
* Declining readership/viewership in both raw numbers and percentage of the population.
* The consistent leftism of these outlets permanently alienating the conservative half of the population.
* The consistent resistance from talk radio from about 1990 onward, led by Rush Limbaugh.
* Since 2010, the rise of Internet podcasts and social media as alternative news sources.
* Attempts by Big Tech and the Biden Administration to censor dissent failing.
The media hasn’t been deserving of the term “Legacy” since “Watergate.” Oh, thirty years ago Democrats were the red states. They didn’t like that and all of a sudden they changed colors, but not their nasty politics.