One, because it looks too much like Facebook and Tumblr had a child.
Two, everything is squished and it becomes confusing.
Three, there are no tabs having the 'Newest,' 'Undiscovered,' 'Popular' tabs on the left side. Plus, people have grown to get used to the messages being displayed in the center, rather than shoved off into the corner.
Four, many ask this question: why fix something that wasn't even broken in the first place?
Plenty of reasons why everyone hates this new update and change to the site.