Probably a cost issue. They already have their mass-production lines setup for the model they're making now... machinery, etc. Also, as said, they make more money selling two items instead of one. Then there are customers who like options... some like the iphone look and others either don't care and want the extra slot or they like the leather look. Also, they might steal some emini customers who come over for the case. What I've wondered about is why they don't make a silver PCC so there ya go.
Edit: Ooops... didn't see there were more posts... answering Eric. Geez... I need to go to bed.. brain fog.