Personal computers are single user devices for general purposes. While the perfomance of a modern personal computer can surpass the old supercomputers, they do not aim for the maximum perfomance. Similarly, the operating system is rather orientated more towards software compatibility, usability, entertainment, office work, mobility, etc. At this moment more than 77% of personal computers use Windows, while around 17% are running macOS (Apple proprietary operating systems). The rest of the devices use ChromeOs or some version of Linux or Unix.
Supercomputers are focused on performing intensive number crunching, mainly for scientific purposes for a large number of users. For a couple of years all supercomputers which have made it to TOP500 have been running Linux.
Linux has many advantages over other operating systems when it comes to supercomputers. Unlike Windows and macOS, Linux is modular and build around a kernel which contains the basic system and drivers. Because Linux is open source, it is possible to customize the operating system for a special high-performance purpose. One can strip down all the unnecessary components and build a lightweight version with only the absolutely essential code. When running in massively parallel scale (i.e. using tens or hundreds of thousands of CPU cores), even miniscule overheads in the operating system can add up and limit the performance.
The license fees of proprietary operating systems are typically based on the number of CPUs, and in supercomputer there are lots of CPUs. As Linux is generally free, it allows also considerable savings in the software costs.