As Scott has stated previously, Jeep is selling as many JK's as they can make right now.
Besides, It's unlikely they would sell very many J8's on the U.S.market. For example, most pre 2012 JK owners complain about the under powered performance of the 3.8L engine. The J8 probably accelerates in the same disappointing manner. But what about the other perks? Sure, it has a D60 in the rear, but those can be had on the aftermarket either new or you can build your own; it's leaf sprung in the rear, if in 2007 Jeep had released the JK with rear leaves, nearly all Jeep enthusiasts would have picketed Jeep headquarters; it's not any more capable off road than a JKU Rubicon, in fact it's probably worse in some scenarios -it has open differentials. It's possible that they may sell a few to contractors, but they don't have the same cargo space as a pickup or industrial van. In all likelihood, they would probably sell 300 or less a year, hardly an incentive considering their success with the current line up.
So what do you really get with the J8? Better fuel economy, torque, a more rugged construction, and a much increased load capacity. What do you get with the JKU? A better ride, ergonomics, agility, 500lb lighter chassis, much quieter interior, lockers, ...and with a ~20% lower price tag. What you do with that extra $15,000 will most likely make you much happier in the long run.
*Just the casual observation of someone who has no actual experience with the J8 (but still wants one too).