Because Linux evolved from UNIX, but Windows evolved from DOS.

A Unix shell can be obtained in Cygwin, a Unix compatibility layer used to compile Unix programs and run them on Windows. Microsoft also makes a shell known as "Windows PowerShell" which incorporates more Unix-like features than the standard command prompt.

Unix is not very popular with the general public. A more popular version is Linux. Around twenty percent tend to run on this system. Unix has competition also.

Unix comes in multiple distributions some more secure than others. Much of the security also comes from how the system is configured. Most unix systems are fairly secure.

Unix is designed to be extremely scalable; I've worked on systems with 12 or more processors, multiple clusters, etc.