Several possible reasons:
1. Most people who buy Challengers in any trim level do so because they like the "retro" look of the original Challenger. I cannot recall any muscle cars from the 70's that had anything other than a black cloth or black vinyl interior.
2. Owners spend a considerable amount of money on their Hellcats and likely take in to consideration what the future resale value may be. Sepia, IMHO, is an acquired taste and may affect future desirability even though the combinations of Sepia with any color makes the car "unique". It's kind of like the desirability of a brown / tan Corvettes, Mercedes, Porsche, etc. Those are pretty hard to sell.
3. All of this being said, I had a 2011 Jeep Grand Cherokee Overland Silver with caramel / Black interior and really loved the car. Unfortunately, a herd of Michigan deer broadsided the left side of the car and I unloaded it after it was fixed. I just bought a 2015 Jeep GC SRT Billet Silver with Sepia Laguna and love the car. It gets many compliments. For me, it's like good Scotch Whiskey, an acquired taste! (FWIW, Sepia was never a consideration for my Hellcat.)