@m3tti because it is fun to use. It is the most interactive of all Lisp. You can test immediately functions. In case of errors, you can see the stack, with the values passed to the functions. These values are fully inspectable: you can navigate in structures and objects and so on.

The effect is that you can easily enter into the zone, and play with your code.

#commonlisp is also an extremely rich language, full of features.