My father, an old clock man, used to tell me that the IIII was used in place of IV because if not, it did not balance with VIII on the opposite side of the dial. Try it, and I think that you will agree that it looks strange and unbalanced. Just look how symmetrical a clock face is.
There may be other reasons too, but this one is OK for me. Regards, Mike Cowham. <[EMAIL PROTECTED]> Cambridge, UK.