ctrl alt del comes from the fact that it is the only keycombernation that can't be sent accross a network
i have no idea why it can't but everyone i have asked says that it can't be sent
probably chosen so that if you remote login to a secure system you can't kill the system
I have a feeling that it can't be put in the keyboard buffer. It became very handy with security with NT logins and access to the task manager, since programmes cannot trigger a Ctrl-Alt-Del (AKAIK). You don't want a virus being able to do those things.
Ctrl-Alt-Del is a very low-level system operation, so it often works when every other key is locked out by a crashed programme. The idea is you can reboot your computer without cutting the power (soft reboot). You may have noticed that soft reboots don't run a memory check.