Freedom

With proprietary content management hosted applications you are locked to your provider.
If at any time you would like to switch to a different provider you will not be able to.
This is not the case with open source where you can download the source and take a data dump of your site and host your content management application elsewhere.
Even more even if you want to change the content management system you can download your database and convert it to any form that your new content management system can read. Open source is freedom.

Why Open Source...