Why do some stackexchange sites such as Askubuntu have their own domain when the rest are subdomains of stackexchange.com? Why isn't it ubuntu.stackexchange.com, which currently acts as a redirect?
At one point, all Stack Exchange sites were going to have their own domain names. This was then rolled back. There has been some debate on the topic over on Meta Stack Exchange. Ask Ubuntu was both grandfathered in and part of an agreement between Stack Exchange and Canonical. This answer by Jeff Atwood gets to the point:
Any way, long story short -- if you can get Apple, Inc. to officially sign off on the site, we will happily change the domain name to whatever is required. Beyond that Act of God scenario, we simply aren't doing custom domain names for any sites as a matter of policy.