There are other reasons that desktops are less popular but in the end fewer people need technical help. Which results in less curiosity about OT and the other social fora here and elsewhere.
I agree with this except that I don't believe less people need technical help. I think it is that technical help is more easily available online outside of forums, such as ifixit and other how-tos you can find with a simple Google search. There was a time when, if the answer to your problem wasn't on the manufacturer's website (if they had one), the only place to go was a forum.
I also think that, as to OT in particular, social networks, like Facebook and Twitter, provide for people what forums use to. I know, as for me, I post things on FB that I probably use to post in OT, but now I can post them to people I know (I know all my FB friends IRL) vs. strangers online.
MotionMan