The reason why Linux has become the only operating system for modern computing is located within the computing history. We have to go back in the early 1980s. In that time, universites and cooperations have bought the newly introduced DEC VAX minicomputer. A typical device was sold for 300000 US$ and provided only 1 MB of RAM. Later generation of the VAX series was called Microvax, VAX cluster and in the early 1990s all the hardware power was put into a workstation like desktop. What remains the same from the 1980s until today is the operating system. The first VAX machines were driven by a Unix operating system and a modern developer PC runs with Linux.
The reason why in the 1980s the VAX computer was equipped with Unix, the later Microvax machine in the late 1980s with BSD, the first Workstations with SunOS and a modern PC with Linux is because without an operating system a computer wouldn't work. Before somebody can install opplications, surf in the internet, and use external discstorage some kind of master control program has to be started first. The kernel gets access to the hardware and on top the kernel user applications can be startet. This master control program is equal to the operating system which is Linux.
The funny thing is, that in 1980s, 90s, 00s and 10s no one has compared Unix with a potential alternative. The situation is, that if somebody decides against Unix/Linux he has no operating system at all. There is no alternative minicomputer operating system for driving larger computers or for powering a developer workstation.
Microsoft has developed a software called Windows NT which can drive a server too. The problem with NT and Microsoft server is, that the system wasn't designed for larger computers in mind. It is not able to power a VAX cluster, and doesn't provide programming tools for example a C-Compiler. The only purpose why Windows NT and Windows Server is sold to the enduser is convince them to buy addtional software which runs also under Windows. That means, Windows is not an operating system from a technical sense, but a marketing tool which puts the customer into a sandbox and restricts his interaction with the computer. For homeusers who want to install the latest GTAV game this might be an interesting option, but for universities who want to educate students in writing software and analyze databases this makes no sense. That's the reason, why Windows NT never was a scuccess in the datacenter. It is equal to Amiga OS a firmware which runs on a certain computer type and is maintained by single company. If the Intel x86 architecture or Microsoft becomes obsolete, the Windows software will decline.
The main reason why datacenters have a need for an operating system is because they are customers from different hardware companies. They have bought the minicomputer from DEC, the printer from HP, the harddrive from seagate and the file storage cluster from compaq. Using a firmware from one of these suply company won't work because the idea to connect all these devices together. This task is handled by a vendor independent operating system. This system is standardized (POSIX) and provides hardware driver for all the equipment. Additional the operating system is the platform for the user. The first user likes to run a webserver, the second likes to program in Java and the next one writes a Python script. The operating system has to handle all these task with the same priorioty. It's not up to the operating system to explain to the user, that Python is bad programming language and he should switch to Borland Pascal. No, the idea is to do what ever the user wants.