Because Red Hat is the default standard in corporates. If you can do Red Hat you should be competent in most forms of Linux.

BTW I did the RHCT exam a few years ago and totally flunked it, it is rock hard for a Linux newbie like I was, even directly after a week-long official training course. Don't even consider it unless you have plenty of experience in Linux and are competent editing Grub configs, disk partitions, etc from the command line. But if you can do all that then the rewards of being a Linux expert are clear.