I can't speak for everyone, but as an Allsparker in their mid-to-late-forties, I am just more comfortable browsing on a larger screen; always have been. Phones are nice and all, but I rarely use them to do much of my online "business of life." Also, this hobby tends to be very pic and video heavy, so I want to see things as large and as clearly as possible.
Edit: I do tend to browse the site on my phone during substantial events, such as Botcon, Comicon, etc., mainly because I want to keep up with the steady stream of news.