Solaris had a lot of useful and unique features, and was backed with a substantial support apparatus. If you developed a feature for Solaris that got in, it'd be documented and supported to quite a high degree.

If you wanted support from the primary vendor of your OS, and wanted a pretty high-quality Unix, Solaris would fill that niche for you. There's definitely a market for that, but not nearly enough to keep Sun or Solaris alive.