Because that's not how clusters work. You can run VirtualBox on each node of a cluster but you cannot run a single instance across the entire cluster.
A little light reading for you:
https://computing.llnl.gov/tutorials/linux_clusters/ https://www.linux.com/community/blogs/133-general-linux/9401
http://www.ibm.com/developerworks/library/l-cluster1/
http://www.linux.org/threads/building-a-linux-cluster.7858/
Because that's not how clusters work. You can run VirtualBox on each node of a cluster but you cannot run a single instance across the entire cluster.
A little light reading for you:
https://computing.llnl.gov/tutorials/linux_clusters/ https://www.linux.com/community/blogs/133-general-linux/9401
http://www.ibm.com/developerworks/library/l-cluster1/
http://www.linux.org/threads/building-a-linux-cluster.7858/