I think one contributing factor to this trend is the fact that full powered desktop and laptop computers have begun to level-off with regard to advances in processing power and design, hence they are not being replaced as frequently.
Mobile electronics are in their growth spurt, with relatively dramatic changes in design and technology occurring every six to nine months. A smartphone can begin looking inadequate compared to new models months after purchase, and after 2 years downright obsolete, both technologically and by the fashion/status influenced standards of mobile leisure electronics. Toss in contract subsidization every two years in many regions, and you don't need to wonder why there's such a big turnover rate for mobile devices.
Meanwhile recent generations of Intel's consumer cpus are coming out with incremental power increases of 5-7%; gone are the days of Moore's Trend. A solid desktop rig can hold its own for 5 years with very little new-tech envy. Laptops maybe a bit less, but you get the idea.