Because it wasn't until of recent that Harley Quinn was drawn scantly clad. For a long time she was originally drawn similar to her first appearance from the Batman: Animated Series.
Harley isn't actually pale skinned, she wears make up. Her clothing style varies greatly from artist to artist. That one you've linked is random deviantArt piece by someone and isn't related to anything. That doesn't prove your point because I could just as easily find one of her that looks completely different. I can find you plenty of varying Jinx arts as well. Also, consider Jinx' skins. Apart from shorts and stockings, she's actually pretty well dressed in terms of coverage. Apart from her base skin, the other 3 don't show any midriff, and her chest is well covered and not accentuated.
And again, Harley Quinn's depiction of 'crazy' varies from writer to writer, but her origin consistently has her as being incredibly smart, but acting against that for a number of reasons (love for the Jokes, manipulated ect).
And the reason it's the same is because you've said 'apart from clothing and a curvier body'. You literally saying apart from the things that make them different, they're the same. And it would be the same as comparing Jinx to Illoai. If Jinx was more muscular and and dressed different, she would look like Illoai. If Illaoi was skinnier and dressed differently, she would look like Jinx.