[jedberg]

Because oftentimes the subdomain is irrelevant and would just make the display cluttered. More often than not the domain would be www.

We did the same thing on reddit for the same reason. A few domains get their subdomain when they are popular enough for people to complain.

[ceejayoz]

No reason www. couldn't be stripped off.

[duggan]

Indeed, it's not like there are an endless number of common subdomains which don't convey much information. "www" is one, "blog" is another. That's about it.