Berkay Mollamustafaoglu manages to hit right at the core of the question of why software vendors are moving to virtual appliances. Are the software vendors better off supporting the virtual machine as the platform?.
Supporting many different platforms (windows, flavors of unix, etc.) is a major challenge for software vendors. One one hand vendors would like to support as many platforms as possible simply because customers demand it, on the other hand every supported platform brings extra support overhead and spreads the resources which is not good for anyone. At the end, the vendors want to focus on their own products rather than dealing with the idiosyncrasies of different flavors of Unix, and customers want to make sure the platform they use has strong support (and they are not the odd ball).