I'm a web designer, so I'll chime in...
It's mainly due to two things. First, a surprisingly high number of people still have what most of us would call low-res monitors. Generally, at the company I work for, we aim for about 960-1000px wide maximum. That way the 1280x800 and 1366x768 people can still view the website without trouble (keep in mind that scroll bars, side bars, etc. take up some of the width, so there has to be a little extra leeway).
Second, we're all used to scrolling down to read more content. If you design a site that's designed to be wide rather than tall, you may run into a situation where the user will have to scroll to the side rather than up/down for more content. That's one great way to piss off your audience - force them to side scroll.
Overall, I think the new design is ok. I would like to see some different colors, but they're just going with the web design trends right now.