Well for one reason, it is probably due to fuel economy. Since the 4.0L is basically a bored and stroked 3.5L, the fuel economy is not going to be as good unless you adjust the gearing which may not help the acceleration in the long term with such a mild power boost, then the second issue is cost. I'm sure a larger version of the same basic engine is going to cost more. It just wouldn't be very cost effective nor really worth it; however, if they were to drop in a H.O. version of that engine like what was dropped into the SEMA Avenger Stormtrooper that produced about 285hp, then it might be worth it.