The incomparable Chris Onstad explains why on his blog:

One thing that’s always made me a bit sad is how Internet presentation seems to devalue content. So much art, writing, and news is suddenly available to us that each piece seems nearly a throwaway, lost in the gullet of our now-insatiable appetite for information. Here in the future, everyone is famous for 15kb. Fifteen reTweets. Fifteen LOLs. Should I work fifteen hours on something that will take fifteen seconds to read? The answer is yes, of course, because I love what I do, but after nearly a decade one wonders if one couldn’t do more for people with that time. Create greater and lengthier entertainment. I’d like to focus more on prose; despite the heavy foot I seem to have planted in the comics world, perhaps I can balance both by shifting the weight a bit. Some might count themselves kings of infinite space when bounded in the nutshell of six panels, but personally I’m finding it a bit cramped.

Best of luck to him—when I interviewed him two years ago he seemed even then to be gravitating toward prose. And while we're on the subject: Obligatory link to the story where Onstad and Patrick Alan Coleman cooked balls together.