Why does this book focus on the programming language Python ?
Of course, there exist many other programming languages. People will tell you that you need some variant of the programming language C (like C#, C++) because everything else is too slow for a computer game, or that you should learn Java so that your games will run inside a web browser.
This is all true and in fact the book will hopefully encourage you to learn as many computer programming languages as you can.
Python is good for several reasons: