Windows was (and still is) the dominant desktop OS. Until recently, it was a very dominant dominance, with competitors usually making up less than 5% of the market share.
Even today, mobile users typically account for only about 10% to 15% of a site's traffic, for sites that don't directly target mobile users. Mobile just isn't as important or widespread as a lot of people think it is.
I don't know wich sites you are watching. All the sites I do control mobile is rocketing away from the desktop. Last year it duplicated. This year looks like it will be higher.
It gives a good sense of the current situation. Mobile browser usage is under 15%. It's nothing compared to the usage of desktop browsers. The growth from 2011 and 2012 isn't even that impressive, given the all of the hype and attention surrounding mobile.
Even today, mobile users typically account for only about 10% to 15% of a site's traffic, for sites that don't directly target mobile users. Mobile just isn't as important or widespread as a lot of people think it is.