It's cliche, but mainly it's because they
just work. The notebook itself is well designed, things rarely crash and if they do it is normally just
that application, not the entire system.
Hook up a monitor and it just works. Plug in a printer and it's recognized and you'll be printing in minutes. Same thing with keyboards and mice, etc. No configuration, diagnostics, downloading drivers from all sorts of different sites.
With VMWare (or Parallels if you prefer) you can run anything you need to in Windows ... with my SSD it now feels like Windows is native - it's super fast. Great for testing web development.
I switched 5-6 years ago and started to get my family and friends to switch. My mom and dad and sister are converts ... bonus there is that I NEVER have to give them tech support. It just works like it's supposed to -- and those that never want to access a command line don't have to, but for those that do it's there when you want it.