Ultimately the economics of a desktop just work better.
You can build an i5-2500k, 8gb ram, 1 tb hdd, gaming caliber video card machine for $600.
Rather than buying a $2500 macbook pro, you can get a gaming desktop with a 30" display, and a macbook air/ultraportable and really come out way on top.
Of course there's the question of juggling 2 environments if you have complex needs, but most people would be fine with a simple dropbox setup or something.
I use a desktop at home and a small laptop on the go, and everything I need is always on both computers. Files, bookmarks, browsing history, you name it. I can just log into either computer and keep working. The advent of cloud-based synchronization tools makes it very easy for people to juggle 2 or more computers. So instead of fighting over the "Desktop of laptop?" question, one can get both and not suffer any inconvenience.
After all, what would the average North American middle-class family do if they needed a vehicle that was both large enough for the kids' hockey games and fuel-efficient enough for the dad's long commute? They wouldn't settle for one mid-sized car; they'd buy a minivan and a Prius.
The proportion of people who are "always on the go" is rather small.
You can build an i5-2500k, 8gb ram, 1 tb hdd, gaming caliber video card machine for $600.
Rather than buying a $2500 macbook pro, you can get a gaming desktop with a 30" display, and a macbook air/ultraportable and really come out way on top.
Of course there's the question of juggling 2 environments if you have complex needs, but most people would be fine with a simple dropbox setup or something.