I can't speak for Cherry Audio themselves, but I suspect it's proprietary code that they wish to keep secret for commercial reasons.
It's the same with CA's 'non-VM' instruments and effects. And of course Arturia plug-ins, and GForce plug-ins, and u-he plug-ins and Native Instrument plug-ins, and......
It's also the same for third-party VM developers - we are under no obligation to make our code open-source. I did choose to go down that route, and made the Dome Music Technologies VMD project files and Java source available to use for any purpose, free from any restrictions.
However, I can fully understand those who choose not to share, and support their decision 100%.