A lot of people are still getting their first smartphone. They already have a Windows laptop or desktop at home. Also, desktops and laptops tend to last longer (for regular users, I'd suggest around 3-5 years) than a smartphone (lifespan of which is usually about 2 years, after which you're upgraded). Finally, a smartphone is "free". Well, not really, but it can certainly feel like it - you can go into a store, pay nothing up front and walk out with a £600 smartphone. People tend to pay up front for desktops and laptops, however.