This appeared originally as my answer to a question on Quora, which can be found here: Why do companies pull their commercials off web?
It is done sometimes to ensure that the new vision and image of the company is maintained, or because of current trends where the previous advertisements may be out of favour/style, or new research may have surfaced which showed that the products advertised are either harmful (or better used for other purposes, etc).
For example, many cigarette companies used to advertise in mainstream media decades ago with many messages that portrayed cigarette smoking as a normal habit for men and women. With current restrictions on cigarette advertising and marketing, and with research showing that their consumption could be harmful, many cigarette companies have pulled down their ads from decades ago. A few ads of cigarettes from way back in the day have been shown here with this answer.
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