Most likely the reason is simple: Journalists[1] are not digital natives. They don't understand or "get" the web. The tools they use don't understand very much beyond print and make linking cumbersome.
[1] and obviously here I'm using a very broad brush -- Ben Goldacre is a good example of a young doctor / writer who really gets the web and there are quite a few others.
The web has very little to do with this in my mind (other than making it even easier to link to sources). References to primary sources should be included whether the article is in digital or print form.
Poor science reporting may be worse these days than ever, but it is not unique to online content.
[1] and obviously here I'm using a very broad brush -- Ben Goldacre is a good example of a young doctor / writer who really gets the web and there are quite a few others.