Kali linux is one of the best penetration testing Linux distributions out there. There are other good ones as well but with Kali linux you get a good balance. If you have ever tried out Blackarch linux you would find 6gigs of penetration testing tools. If you are new to penetration testing you would soon find loading up 6 gigs of tools and not knowing what they do very overwhelming.
Kali linux is good at what it does but one should not install it. It is not a general installation and if you are new to linux you will soon find that installing Kali linux as your main operating system is not what it was intended for.
Certain Linux distributions are designed for specific tasks and some of them are really good at what they do, problem comes when you try to use a specific distro for something it was not intended for.
For people new to Linux distrobutions like openSUSE, Fedora and Debian based ones would work better, maybe Ubuntu or Mint.