User blog comment:DarknessStriker/Which support should I choose?/@comment-9048637-20130520180835/@comment-8506165-20130522063801

In theory there's no reason why Thresh can't jungle - he can definitely clear, and the large jungle monsters always drop souls. It's just that his ganks do depend largely on him landing his Q, although arguably once you hit 6 even if you hit a nearby minion pulling yourself over and Boxing them should still result in a pretty successful gank.

It probably just comes down to him being naturally best in the support role though. He's the only champ who can get free stats without CSing, which would be a pretty big case for him being played support even were his kit not so perfect for it. So even the more obvious other roles he can do, like top-lane offtank, are rarely seen because it means you lose out on support Thresh.