Some Virtual Instruments Can't Use Instrument Tracks

[digijohn]

Hey guys -

This isn't the end of the world or anything, but could someone explain to me why some virtual instruments can't make use of the instrument tracks in Pro Tools?

I'll give you a fer instance:

I use Trilogy and SampleTank LE. If you create an instrument track and activate these plug-ins, I can't trigger them via MIDI. Now, if you do it the old school way by creating a stereo audio track then create a MIDI track to trigger it, it works fine. What gives?

Other VI's work on an instrument track just peachy, like Strike, Stylus RMX, Kontakt, etc. Is it because the other plug-ins are behind the times?

Thanks for any insight.

John