For many years, almost every one used Internet Explorer from Microsoft, which had 95% of the market at one point. The IE market share is around 65% today and much of that has been taken by browsers like Firefox.
Early in the IE8 design process the development team had big plans for privacy setting on the browser. The settings would have been integrated in a way that would have seen almost all users of IE8 completely blocking tracking software online.
This didn't sit well with the ad execs at Microsoft who saw it as a big hit to profits. The ad team won out and the security features turned into InPrivate browsing and are rarely used thanks to the fact that users have to turn the feature on each time the browser launches.