At one time, most of the world’s dedicated servers used a form of UNIX. Big mainframes were known to have big expensive UNIX installations with hefty, costly licenses. Many of those servers have gradually been phased out or have been switched to more modern operating systems like Linux or BSD. Still, some persist, and those are at a serious disadvantage.
UNIX is not open. Unlike Linux and other free and open source OSes, UNIX is closed, and once the companies that created them stop supporting them, you will not have access to the source code or any rights to it.
UNIX is slow. It generally has not been optimized for today’s faster, more energy efficient servers. HP-UX, AIX, and other Unices are relics of the past.
UNIX is expensive. It was expensive to license and will continue to be expensive to maintain, as you will likely need specialists to manage it. Many of today’s up and coming system engineers have been taught to manage modern operating systems, not old UNIX machines.