Before the rise of the Internet, traditional media like magazines and newspapers got the majority of their revenue from advertising. It's nice when people pay money to subscribe but it wasn't enough then. It's probably not enough now.
The advertising will continue. The subscriptions are only meant to define the audience demographics. Lots of print media companies actually limit their audiences to only marketable demographics, especially magazines. Magazines often say they're oversubscribed if they don't hit certain demographic profiles, so their advertisers aren't being wasted. For example, if a bunch of poor college males were subscribing to Vogue, it doesn't help their advertisers. And those subscriptions costs a ton of money in mailings, so they tighten their audience to be core advertiser friendly.