NOTE: as of 02-04-14. Canonical has announced that they will be shutting down the ubuntu one service, and in the process, will make the server side application open source as well, as detailed in their blog post
I've come across the Launchpad bug report which gave me some insight into people's opinions on the matter, but didn't explain why Canonical made this decision. The Wikipedia article on Ubuntu One didn't contain any links to announcements that would clear this up.
Question: Why is the Ubuntu One server side application proprietary?