Yeah that's true, they didn't exactly stop promoting Wario's greedy personality in the Ware games. But I wonder who at Nintendo is responsible for promoting it? And if the people at the top are really all that fond of it (not that they hate it, either, but like you said, it's their black sheep)?
Also, another thing that's keeping the Wario Land series down is the competition it has with all of the 2D platformers that started coming out after the 2D platformer revival. Remember, during Wario Land's heyday its main competition was ports of Super Mario and DKC on the GBC and GBA. Whereas nowadays there are a ton of genuinely awesome 2D platformers coming out on a fairly regular basis, and not everyone is enough of a platformer aficionado to want to buy them all. And with New Super Mario Bros hogging up so much of the fanbase's attention, every other 2D platforming series is basically clawing for what's left over.
Also, another thing that's keeping the Wario Land series down is the competition it has with all of the 2D platformers that started coming out after the 2D platformer revival. Remember, during Wario Land's heyday its main competition was ports of Super Mario and DKC on the GBC and GBA. Whereas nowadays there are a ton of genuinely awesome 2D platformers coming out on a fairly regular basis, and not everyone is enough of a platformer aficionado to want to buy them all. And with New Super Mario Bros hogging up so much of the fanbase's attention, every other 2D platforming series is basically clawing for what's left over.