Most of the content online is written by amateur writers (including yours truly). It's easy to see why. The barrier to publish on the web is so small. All you need is a device, bandwidth, and 15 minutes to create a free account on blogger. In comparison, how difficult is it to get a book published and distributed?
There also isn't a lot of money in writing online, where distribution is free and ad revenues are still pretty weak (compared to newspapers and magazines). Consequently, you don't often find online content that has been carefully researched, edited, reviewed, revised, polished and published. Publishing online tends to be hastier because few are willing to make that investment.
These two facts create a situation where there is an enormous amount of material online. A lot of it is good, but very little of it is truly exceptional. With so much to sort through and so little to find, the opportunity-cost of reading a long article is very high. So we don't read long articles online.
We surf. We skim. We sample, and we move on. We're like children on an easter egg hunt, checking a spot for something good, making a snap judgement1, then moving on to the next place.
This behavior has been researched and documented. Only a small percent of visitors read online articles to completion. Maybe it has something to do with the intangibility of it all. In the physical world, the fundamental unit of content is a book. You can hold it in your hands, keep on your shelf, or give to a friend. Indeed, many who write on web topics (like these guys) have taken their writing off-line in an attempt to make it more tangible (and more profitable).
Online, the fundamental unit of content began as a site, then a page, then a blog-post. Now it's a tweet. It's hard to for somebody to only read half a tweet and move on.
So what do you do if you're writing for the web? I'm not sure I have all the answers, but one thing I know for sure. If it's going to be long, you better make sure it's REALLY good, or your readers won't stick around for the punch line.
 Note, this highlights the importance of good web design. People will only spend a second or two looking at your page, and if anything they see indicates "low-quality" they'll be out of there.