This choice of letter is not a big deal. It was the choice of the author of
the game to call the new piece a Nuclea. Feel free to call it a Bomb if
you like. Bishop is a Fool in French you know, so the names of piece have
demonstrated some flexibility.
Btw, Knight (which does not start with a N, no?(humour)) is Cavalier in
French, so we use a C.
And, it took me few seconds to get what ICBM meant...