The best reason, IMO, is that the tools around KVM (using QEMU) and virt-manager are becoming much more pleasant to use than their VirtualBox counterparts. The

The best reason, IMO, is that the tools around KVM (using QEMU) and virt-manager are becoming much more pleasant to use than their VirtualBox counterparts. The GUI could still use some cleaning up, IMO, but overall I have found the experience much more pleasant in recent virt-manager releases than the VirtualBox way of doing things.