Because the community of contributors has on multiple occasions strongly objected to advertising, on several occasions resulting in fracturing. In 2002 when th

Because the community of contributors has on multiple occasions strongly objected to advertising, on several occasions resulting in fracturing. In 2002 when the idea of banners ads was seriously raised the majority of the Spanish Wikipedia contributors forked and started Enciclopedia Libre, despite the fact that Wikipedia decided not to go ahead with adverts it took two years to attract enough contributors back to rival the forked site. That's not an experience the Wikipedia community want to repeat. Back in 2004 I proposed that Wikipedia made opt-in ads so people who wanted to support Wikipedia by watching ads could do so, but there wasn't enough support for the idea to gain traction.