About when the internet started taking off as a consumption medium I think. Desktop computers started to show up in every home and were largely there just as a gateway to the internet. Prior to that, they were mostly for office work and computing enthusiasts with the occasional non-computing-enthusiast gamer thrown in. Since the consumer market was much, much larger the focus shifted.
The strange thing to me, though, is that once smart phones and tables took over as the preferred platform for internet consumption the desktop OSs didn't start reverting to targeting the market that still wants them. Instead they doubled down on trying to turn desktops into smart phones.
