I made the switch because I no longer had the time and interest in maintaining my OS - I wanted it to just work. Much can be said for Linux distros' improvements in hardware support over the years, but it pales in comparison to paying for a superbly built computer (even Torvalds uses an MBA) and never having to think about hardware compatibility or OS breakage again. That's worth the (significant) premium in cost for me, and it is worth it for many others on HN.
(This is only on the desktop side, obviously; I use an Android phone and run Debian/Ubuntu on my servers.)