- Desktop devices tend to last much longer before being completely replaced (unless you're a gamer). Even if you change your desktop machine, you often keep your keyboard/mouse, external NTFS drives, etc.

- There are more platform-specific apps that can tie you to a certain OS, either because of the app itself mat not be available (Games?), because you'd have to buy it again (Photoshop?), or because of the investment in workflow (MS Office?) or data (iPhoto?) that you have already made for it.

Once a mobile platform reaches a reasonable level of availability of popular apps, it's essentially viable for anyone to switch to. Unless you are addicted to Hay Day or rely a lot on Paper, or something like that, it's easy to move to a new platform. That's why it's also crucial that aspiring platforms make it totally easy to port stuff, and here's a place where Microsoft has made a huge mess with all their awesome but somewhat restricted tools, and constantly moving goalposts and platform strategy (C#? .NET? Silverlight? XNA? WPF? WinRT? HTML5? C++? Native or Managed or /CLR or...? I don't even know anymore).

All that said, OSX and Macs are much more popular now than say 5 years ago, so there is some ship jumping. The cloud really is eroding people's attachment to they desktops, and that's the reason Chromebooks and even Linux have even a chance at reaching the mass market.