I'm not an industry expert, but considering how there have been so many stories about the death knell for PCs (what with tablets and Chromebooks rising to power and Windows 8.1 floundering), and how Sony seems to have really reinvigorated their mobile device lineup, my guess is that they just want to concentrate on what they feel is the future of tech.
Who knows, maybe we'll see a Sony Chromebook in the next year or so?