"To make matters worse, the big new players in the Unix market promptly committed major strategic blunders. One was to seek advantage by product differentiation — a tactic which resulted in the interfaces of different Unixes diverging. This threw away cross-platform compatibility and fragmented the Unix market." - http://www.faqs.org/docs/artu/ch02s01.html
It's what allowed IRIX to be used to perform graphics-related tasks that basically couldn't be done before, or on other systems.
It's what allowed SunOS and later Solaris to become extremely powerful workstation operating systems.
It's what allowed AIX and HP-UX to excel as server operating systems.
Only within the past few years (a couple of decades after the so-called "UNIX wars") have we seen Linux getting to the point of being a single system that can cover such vastly different areas sufficiently. Even then, the experience still isn't as smooth as it was on more focused systems back in the 1980s.