Why demo.bpmn.io is not open source?

BPMN-js have many good features but documentation is very weak.I want to use this library to create modeler in my project but for add, each feature .i will try some different example with different ways.
demo.bpmn.io is a very good example but is not an open source.
why do not make open source this project?

You can access the source code using your browser developer toolbar.
For example, on Google Chrome

Our modeler example is essentially the demo.

I do not see what you’d gain from OSSing the demo.

And what you gain from not OSSing the demo ?
for example why jhipster make open source start.jhipster.tech ? what developers gain from that ?