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Probably because despite their many disguises Team Rocket have been wearing the same uniform for every series. When they got the black outfits there was a boost in fanart because Team Rocket finally had a new look. Also her disguises rarely showed off her figure so most fanart of them were more for the cuteness factor (DP contest clothes for example) than how hot she looked.
I mean there were exceptions (AG contest clothes) but new girls tend to still get priority. Not just the player characters and Ash's companions but Gym Leaders. I mean Jessie had some hot outfits in Advanced Generation but not enough to overshadow Flannery and Lucy.
