Legend says that back in the mists of time, when the first UNIX was created, its developers needed a low level command to copy data between devices. As they were in a hurry, they decided to borrow the syntax used by IBM-360 machines, and to develop later an interface consistent with that of the other commands. Time passed, and all were so used with the odd way of using dd that it stuck. I don't know whether it is true, but it is a nice story to tell.