Web design has become simplicity itself. Why?

Because we like it that way, or so says Designersmind.com

Why are these complex designs no longer popular? Although designs like this do have their place, the truth is that the internet is going back to basics. Your average internet user isn’t coming to your site to see your robust flash intro, and they probably won’t stay because your site looks pretty. They’re there because they need relevant, quality content, and they want it fast.

On the other hand, is there not a long tradition of simplicity in design back as far as modernism?

Moreover, graphic design moves quickly and trends come and go. Scratchy freehand illustrations were popular a few years back, followed not long after by floral motifs. The DIY punk aesthetic, aesthetic, regularly comes round every few years.
The moral of the story? There isn't one. On the other hand, the is still something of a tension on the web between design and usability and that is something worth looking into.