In the beginning of time, the one thing that stood in the way of competing with the IBM PC was the BIOS. Someone hired some machine-language programmers who had never seen or used an IBM PC (possible back then), told them the inputs and outputs that they needed from the BIOS and had them reverse engineer it. After that, there were several BIOSs
Ctrl+Alt+Delete was built into IBM's BIOS as the key combination to restart the machine, so it was built into every BIOS. When Microsoft began the NT operating system (marketed as NT, 2000, XP, Vista, and 7), it was the first time that computers had more than one account. Before about that time, there was no logon, not even on a Mac.
Windows NT had user accounts and needed a way to logon securely on PCs that were already in existence. Microsoft did some experimentation with different key combinations, and they found that intercepting Ctrl-Alt-Delete was the only way to do it. It was the only key combination that was embedded in the BIOS of every manufacturer's computer. It was ideal: it worked on every PC in existence and did not require manufacturers to make changes to the BIOS. Microsoft didn't invent it, they figured it out.
Today, you log onto Windows with Ctrl-Alt-Delete, but if you press it twice in a row, it performs its original function and the PC reboots.
Ctrl-Alt-Delete was the way to restart the computer from the keyboard. It requires you to press keys on opposite sides of the keyboard simultaneously, so that you are unlikely to restart by accident. The Mac equivalent is almost the same: Ctrl-Cmd-Eject restarts a Mac.