VST is not an official standard like MIDI. It is something that was implemented in Cubase to control audio effects. It was adapted by others "big ones".

Making software able to host VST will cost a lot of programming effort. That may not be a problem for sequencer and non-real-time arranger software wich is a multi million dollar bussiness, but it is not worth it for real-time arrangers that are in a penny and nickle market.

Jos