One explanation I read (I believe in the Star Trek Encyclopedia) is that the creature from The Savage Curtain created the opponents from Kirk and Spock's imaginations (just as he did their allies), and since Kirk and Spock knew only Klingons of the TOS variety, then that is how they imagined Kahless. Sorry for the lack of sources on this one.
The original Kahless, having lived eons before any series takes place, was therefore of the non-smooth forehead variety, and his clone remained thus. The thing that's nice with this explanation is that whether or not you consider the Enterprise retconning, it works.