Unix was created at AT&T employees working at Bell Labs. A lot of these employees have been involved in design of MULTICS, which was short for Multiplexed Information and Computing Service. Back then, mainframes were so expensive that users had to operate by time sharing mechanism where they connected to mainframe by terminals in their time slots, which had a lot less computing power than mainframes.
MULTICS became so huge and complex that Bell had to pull out of it. But people in the project, created UNICS, simpler and smaller version of MULTICS as a side project. It was short for Uniplexed Information and Computing Service. Very quickly, it became able to support multiple users and got name UNIX.
At that time, UNIX became just another mainframe operating system used internally by Bell Labs. Two events led to its popularity. First is, it was rewritten in C. C was already programming language created to write programs for Unix. Back then, operating systems were written in Assembly which was specific to hardware. It is very fast but not portable. You had to rewrite most of operating system according to the target architecture again. Rewriting Unix in C made it portable.
The next event is the free distribution of Unix by AT&T thanks to anti-trust laws. AT&T could not sell the Unix OS so they distributed it freely. Distribution also included source code. Government agencies, corporations and most importantly universities who had mainframe computers could get a free license and source code to be able to run on their mainframes.
So Unix became very first things programmers learnt back in the day. First release in 1975 was called System Five. That is the first public version of Unix. Over time many branches and improvements were published and most famous of them is Berkeley Software Distribution or BSD that came out in 1977.
Many other versions of Unix came out as open source, closed source or mixed source. Linux is open source. Closed source versions are Sun Oracle's Solaris, IBM's AIX etc. Mixture of open source and closed source softwares also came out, like Mac OS X.
So, no one is using pure Unix, or System Five. All these operating systems used today are Unix-like systems. Mobile devices are Unis-like systems too since IOS and Android is based on Unix. Mac OS X is combination of BSD, NextSTEP(Next is a company found by Steve Jobs) and Apple Code, which is called Darwin. Of course, Mac OS X is not only Darwin, it just sits underneath Mac OS X.
Why do we use command line to use Unix directly instead of GUI. First of all, it is more powerful. It can be thought as automatic transmission vs manual transmission. You are gaining control and losing convenience by using command line over graphical user interface. More power to the developer!