Part of it is wheelbase...the new unlimited Wrangler, with it's longer wheel base, is rated for 3500lbs. On a stright line, no problem towing that with the "normal" Wrangler, but when you turn the trailer tends to want to keep going straight (or flip the jeep).
Truthfully, Jeeps aren't great tow cars for heavy trailers..can yank them out of a ditch, but not the best choice for towing a large heavy item down interstate.