1. Linux is open Source and any one can use 3/5 for any type of work, not only for h*cking.
2. Most of the linux Distros are free of cost. One of the most common linux distro is UBUNTU, followed by mint, debian and others.
3. You can edit, copy, distribute various aspects of a linux based OS without violating any copyright law or term and condition. This the reason I think, H*ckers use linux, because they can easily develop softwares used for hacking and pen-testing.
4. Linux OS like kali linux, parrot, backtrack are especially made for pentesting and checking security loopholes, often used for h*cking also.
5. They are made and maintained by developer forums, and are always available to fix bugs, you just need to put your problem in the forum and your problem will be discussed by thousands of developers.
6. They are secure and virus free, no antivirus needed.
7.Easy to use. There is a widespread myth, that linux is only for programmers and h*ckers, which i believe to be wrong, I have been using it for years and I am not a veteran programmer or h*cker and I found it more convenient and user-friendly than windows.