The URL for an article is set permanently when you set the title the first time as mebel says. This is because you might get links to the URL, Google will index it, etc
If the URL were to be editable, Hubpages would need to set up 301 redirects for each change so that the old URL redirected in a correct manner that passed pagerank to the new URL. Logistically this would be a nightmare, especially if you can keep editing the URL. Hence, they make the URL permanent.
It's no great loss. While domain name is considered to have a role in SEO, filenames (the URL) aren't these days.