They aren't exactly the same thing, I suppose, but there's enough functional overlap that if you already have one, it's hard to justify adding another. You don't want too many of these "escape sequences", because every kernel-side keyboard interrupt becomes a thing that:
* Userland doesn't get to use that keyboard shortcut any more.
* Someone might accidentally trigger, and if they don't know what's going on, they become stuck.
* It's another thing that anyone building a captive kiosk has to know how to disable.
It can also be challenging to actually input a magic-sysrq combo on many modern keyboards where sysrq isn't its own key, if it's labeled at all as some Fn-key contortion.
Ages ago I worked with a small team of young sysadmins who would often cause trouble for one another abusing things like sysrq, or poorly configured XFree86 instances without DontZap enabled (ctrl-alt-backspace termination, to a shell when startx/xinit is being used). Most of the time nothing malicious was actually done. But when you return to your screen-locked X desktop to find a fullscreen VC with a root shell blinking back at you and an empty history, guess what you'll at least have on the back of your mind for the rest of the workday?
It'd let you safely shutdown when the computer is locked up otherwise.