Well, I'll tell you why. It's because if Jeep rated the Wrangler for what it's actually capable of towing, it would be poor business. Each model in the Jeep lineup has to have unique selling points. If the Wrangler and the Grand Cherokee had the same tow ratings and the same physical size, it would hurt sales. As it is, they can market the Wrangler as the offroad model, the GC as the luxury and heavy towing model, and the Liberty as the compact and capable model.
Truth is, the Wrangler mechanically is the best towing vehicle in the entire Jeep lineup. Ladder frame, solid axles, Pentastar engine, deeper gearing options, etc... it should have the highest towing capacity available in a Jeep but you'll never see them label it as such.
Bad answer and probably the stupidest thing I have ever heard. Why kill sales of one line when having TWO lines would DOUBLE sales as BOTH lines see equal interest and pull from other brands? That would be like McDonalds killing all but one hamburger, one cheeseburger, one chicken sandwich and one fish sandwich. Ford has been kicking everyones rear in Truck sales (not just bragging - the F series has sold more units than anyone else throughout history) simply because every member of the F and E family has a HIGHER rating than the average counterpart in the competing brands.
The SAE's Surface Vehicle Recommended Practice J2807 spells out in precise terms a procedure for determining two important ratings: the maximum permissible gross combination weight (GCWR) for a tow vehicle and its trailer and the maximum permissible trailer weight rating (TWR).
There are five engineering characteristics that strongly influence any tow vehicle's performance and rating under SAE J2807:
The engine's power and torque characteristics.
The powertrain's cooling capacity.
The durability of the powertrain and chassis.
Handling characteristics during cornering and braking maneuvers. (direct result of suspension design).
The structural characteristics of the vehicle's hitch attachment area.
SAE J2807 spells it out nicely. Probably the biggest thing holding the wrangler back is the dual 4 link, opposing track bar, live axle design. Although best for crawling rocks the low roll center (ground level) coupled with the high center of gravity (about 26" above ground front axle line and 18 above rear axle line) makes a HUGE roll moment - and a momentum line that is sharply lower in the rear!. This would make a heavier trailer way more likely to tumble that tow rig when hard on brakes and hard in an evasive maneuver. The Grand has non parallel control arms (roll center probably about 5" above ground) and lower CG (about 20" above ground) thus a shorter roll moment AT BOTH ENDS and the liberty is just below that (even a pickup with 4 link live rear and cntrl arm front has a MUCH lower roll moment line - although leafs (roll center about even with axle line) rear and ctrl front - (especially Fords twin I Beams with a roll center that moves upward with added weight) is ideally the best as then the moment line angles down in the front!).
To simplify - roll center is the mechanical axis upon which body roll occurs and is dictated by suspension design and geometry - center of gravity is the application to the lever created by the distance between the two points (the moment). The longer the distance - the longer the leverage (moment) the more the vehicle wants (roll momentum) to body roll (lean) in cornering. (the roll moment front combined with roll moment rear creates the roll moment line) The higher the roll moment line the lower the towing capacity per SAE (all things else being equal) - PERIOD. Roll is noticed as when you turn right the car leans left - the weight passing over the roll center! (if an indy car could tow it would be awesome - their centerline of gravity is below the roll center - so rather than the weight passing OVER the roll center and the car rolling opposite the steer - it passes UNDER so the car rolls INTO steer - banks left when turning left and banks right when turning right - but with weight and suspension travel that design would get very scary once you are seeing more than just the fraction of an inch negative roll momentum they see)
Sales, marketing and brand or model favoritism are nowhere in SAE J2807 - the engineering standard that sets what rating is given any vehicle - it is simple math and calculations followed by testing at the proving ground!