At the time (1993, when NT came out) there was a good reason.
C-A-D was a special reserved keystroke that couldn't be intercepted or redirected. So you could use it even to interrupt a task that had frozen the entire UI.
Using it for login meant it could bypass and circumvent any malware that attempted to steal credentials, etc.
Then MS let people replace the GINA that handled it, and then they let people turn it off, and it joined the rest of the slide into junkware...
