Why would a corporation desire to run many apps on separate virtual machines rather than a single server?

The benefit of running programs on their own virtual machine is that if the security of one application or VM is breached, the others may still be secure. Scalability would still be constrained by the server's resources. The hardware maintenance would be the same, but the security maintenance could be greater; nevertheless, the ability to set up different access for each machine could ease security settings. The cost would really be higher because each OS running on its own virtual machine would require a separate license.