Because you'll be very hard pressed to find a light enough fifth wheel that you can tow while being within the rating of your truck. Most 5th wheels are heavy pigs, and then put 20% of the weight of the trailer on the back of your truck, plus the hitch, some cargo, and some passengers, and you'll blow right past your payload.
Simple example; 9000 pound 5th wheel at 20% tongue weight = 1800 pounds. That's just tongue weight on an empty 5th wheel, and that's already beyond the payload of most 1500's unless you get a v6 tradesman. At that point you haven't added any cargo to the trailer, or the truck, or added any passengers.
A 9000 pound travel trailer at 10% tongue weight = 900 pounds. Already creeping up quite close to 3/4 payload just by hooking up an empty trailer. But doable if you're careful.