We’re a left-to-right kind of culture, in general. Cavemen painted left to right and Medieval artists wove tapestries left to right. But at some point, the Internet began to grow vertically — into the feed mentality that dominates almost every website. If you have a scroll wheel on your mouse, it goes up and down. There's a reason for that.
I'll agree that thanks to the web and all the real time streams we are using nowadays, a vertical view feels more natural for some stuff. I often hold my iPad Air in portrait mode when going through my Google+ and Twitter streams for example, but I always use it in landscape mode when doing video editing. So we still need both modes. But websites and web surfing fits better in portrait mode. That's one more reason why I love using my iPad Air as my default environment now, it's so easy to use it in both landscape and portrait mode to fit the task you are doing at the moment.