In general terms, the price you pay for a subscription to a periodical covers the cost of getting it to your door, but not what's inside. The ads pay for the content.
The problem in that as periodicals reduce the cost of delivery by switching to electronic versions, the value of their ads drops too. While delivery becomes profit (no real cost of pushing data) revenue drops on the ad side.
The result is a revenue loss, thus upsetting the model.
Creating content that's compelling enough to subscribe to the electronic version is the Holy Grail.