No one thinks he sucks, there are junglers who are just better in virtually every possible situation. A lot can gank way better pre 6, his great sustain isnt that important because the jungle is currently (not S3 yet) very easy for your initial clear. Against a good team who uses wards, and doesnt put themselves in terrible positions your not going to get an opportunity to gank pre 6. This means your going to behind every laner on gold, so your generally going to have to go into a support build. There are junglers with more utility, who benefit your team more then WW from gp5 items, who then end up going tanky/support and are just so much better then a ww with just his ult. Such as amumu, maoki, Hecarim, and Cho. There are bruisers who are better in team fights such as Malphite, J4, Skarner.
Ya warwick can solo baron at 18 but as a jungler he isnt going to get the gold to build that way ya you might once every 10 games but its just ineffective to build damage as a jungler when your gold starved. Solo top gives more gold then jungle WW needs that gold to be most effective.
Warwick is great when he gets stupidly fed but feeding your lanes is more important warwick isnt good at that.