It's also a kinda crappy alternative, because it's not possible on a large number of non-English keyboard. Ctrl+c is also an option, although I think there's a slight difference.
ESC is 0x1B, traditionally the CTRL key reset the two highest bit of the ASCII code, [ is 0x5B, if you reset the two highest bits of 0x5B you get 0x1B which is why ESC and CTRL-[ are the same thing.
CTRL-C is completely different.