Competing hypothesis: advertisers irrationally spent money on advertising, because they had beliefs about effectiveness and reach.
Internet advertising comes with much better feedback mechanisms than ever before possible, destroying those irrational beliefs.
Secondarily: advertising quality has generally been correlated to price. Internet advertising reduces the price so far that we don't just get the equivalent of a local used-car dealer in between the national toilet-paper ad and the international fast-food restaurant ad, we get a Nigerian 419 scam and Ed's Fantasy Football League afterwards.