That ESC is to allow more control characters to be defined than what was initially put into the standard. Consider for instance how ANSI colors are written to the terminal. They are escape sequence control (ESC? I'm not sure if the original designer meant to be meta) characters, identified by the escape sequence (ESC)[.
I'm pretty sure that's later development. Otherwise it would be called "prefix" or "extend".
Unfortunately I cannot quickly find a better source than:
"The "escape" character (ESC, code 27), for example, was intended originally to allow sending other control characters as literals instead of invoking their meaning."