Second, Oracle is encumbering Solaris with a not-nice license. I haven't followed the latest developments because I no longer care, since I will never use a non-free OS.
Whether you meant to ask, "Why Illumos over Solaris" or "Why SmartOS or OmniOS over Illumos?" I think I have answered your question :-)
Hopefully the next release of OpenIndiana will add support for it as well, seeing how they also use the Illumos kernel.