I think you mean Stack Exchange? - http://www.stackexchange.com/
If so, the main reason is that I think a hosted service is the wrong model for this kind of tool. Serious websites want (a) the data in their own database, (b) to be able to see the underlying code, (c) to customize it as and where required, (d) not to have to pay based on usage.
By way of comparison, most big sites use server-installed forums (like phpBB or vBulletin) rather than externally hosted ones. The same goes again for content management systems, e.g. WordPress, Joomla, Drupal.
The final problem with Stack Exchange is that it's built on Microsoft technologies, so even if they did decide to open source it, it wouldn't be appropriate for mass market adoption. Being built on PHP/MySQL (which are themselves free/open source) is key to making Question2Answer appeal to as many website owners as possible.