Remember, too, that a Jeep or any other offroad-capable vehicle really needs a bigger, low-rpm, high-torque engine for offroad use -- you have to be able to do a slow crawl over large obstacles.
The 4.0L inline 6 was good for offroading for that reason, and it was cheap to produce. It's really 60's technology, though, with a real good multipoint EFI system added and some minor refinements otherwise (lightweight aluminum rods and pistons, roller rockers, roller timing chain, improvements to the combustion chamber and elsewhere, etc.). But it's still old tech. That engine will be out of production very soon. They phased out the Cherokee and replaced it with the Liberty, which uses a much more efficent V6. (Of course, the Liberty weighs about 600lb more than the Cherokee, so they ended up with little or no better gas mileage as a result, go figure.)
That 4.7L is much newer design from the oilpan up, so that helps to offset its larger displacement and give it practically the same fuel efficiency. Since the second-gen GC is larger and heavier by a good amount than the first-gen, that makes it tough, too.
I imagine the other models like the GC will get that new engine to replace the 4.0L completely.
To hell with fuel economy -- give me that new 5.7L Hemi engine in a Grand Cherokee!