Because it is a Unix system, so the Bash feels very similar and if you install homebrew you have a package manager with a very Linux like feel and a software catalog where most Linux command lines tool have a port for OS X. From a usability perspective OS X is Linux, but without permanent issues and reinstalls. (and I am saying this as a 12 year Linux User who only uses Macs for work)
Because it is a Unix system, so the Bash feels very similar and if you install homebrew you have a package manager with a very Linux like feel and a software catalog where most Linux command lines tool have a port for OS X. From a usability perspective OS X is Linux, but without permanent issues and reinstalls. (and I am saying this as a 12 year Linux User who only uses Macs for work)