In the past, media have always depended on limited access to channels for at least a portion of their success. Each media vehicle has to hire and maintain staff to prepare content, make sales, etc. Put 100 channels in a market of 1 million people and the channels will average 10,000 each. After a while everyone starts to think that 10,000 audience is significant. Attracting more than the average audience makes the channel a winner.
Losers fall below the average audience size.
Limited channel space also affects the sales process. Agencies are paid by the amount of advertising they place. In a world where advertising runs on specific individual channels, advertisers have to find a way to limit the discussions that they have with salespeople to make the process as efficient as they can. Agencies therefore look at things like rankers to rule out channels which fall below some arbitrary number. In fact, you could argue that niche-appeal stations actually have more loyal audiences who may be more responsive to an advertiser's message.