Solaris *is* UNIX

Probably one of the only non-UNIX OSes besides Windows which got recognition other than in specific technical fields was
BeOS (along with the old Mac OS).
I'm not technical enough to give an insight on the exact reason why on a professional comp sci level UNIX is highly regarded... but based on my personal day-to-day experience with it, I'd say it has a lot to do with the simpleness of it. Everything that isn't a binary is an editable text file. The foundation of its userspace is based on the integration of several smaller programs that, together, can manage to accomplish very complex tasks. There's more detailed information
here.
I'd recommend you to read
"In the beginning, there was the command line", an essay by Neal Stephenson written in IIRC 1997 (who then backtracked on several of his points when OS X came about, but that's another story). The essay is interesting on a technical level, but it also conveys a very clear social critique which I found fascinating.
Keep in mind that the Mac OS he talks about there is the old one, which wasn't based on UNIX and was quite crappy when compared to Windows.