VirtualBox is available and just as good as elsewhere. Giving everyone the benefit of the doubt and assuming all decisions were made purely on merit, I imagine that Apple might prefer to push Parallels because it does a lot of work tightly integrating the two environments. For example, file associations work in both directions, from the Finder onto the Windows desktop and vice versa, the start menu is added to the dock, your Windows applications appear in /Applications and hence in the Launchpad, etc.
It's actually why I stopped using Parallels and started using VirtualBox. I needed Windows for a particular application only and otherwise to be contained in its box — I actively didn't want Parallels to start messing about with my whole system.