Part of me thinks it is all the modern tech that has shattered my attention span. I'm too used to seeing something new, refreshing a page, opening a new tab, browsing my twitter feed, etc, that focusing for too long is difficult. This is covered in the article as the first reason, and I think it is the most compelling.
Lately though, I've started rereading some of my old favorite books and I've been pleasantly surprised to find my attention holding up better. As I have less time to dedicate to reading now, I think it's more important to focus on finding good books - and one way to know you've found them is that it isn't a struggle to read them, but rather a struggle to stop.
What success I have with continuing to read has been rereading old favorites to build the momentum and focus back up, and sampling lots of books, finding good recommendations, and not hesitating to give books up if I don't like them or if I think I already understand the point. If I have limited time to spend reading, I'm not going to waste it reading stuff I don't like.