The reason why is a matter of dispute.
The ads showed Gates and Seinfeld trading banter at a mall shoe store and while living with a suburban family, trying to get in touch with regular people.
Bloggers and online media have suggested that Microsoft yanked the Seinfeld ads after they were poorly received.
However, Microsoft says it was always its intention to retire the bizarre two-week-old campaign and replace it with ads that focus on Windows.
The Windows-focused campaign will attempt to turn Apple's "I'm a Mac" ads on their head.