because goth lolita clothing are in fact not a western invention; it is western derived/inspired.
it's still called western by japanese becuase it's made in a western style, draws inspiration from it...
the reason why lolita is considered western while katsudon is not (fried meats are a...
the western world to most people means Europe - Russia + North America - Latin America I believe
some people (mostly Asians like me) include Latin America and Russia into the concept of western but that's usually just in the context of food (Tacos and Stroganoff are usually considered western...
I'd argue that usagi drop's final few chapters which tarnished it's image of being a wholesome family manga had a massive impact on any manga depicting a dude and a loli in a somewhat father/child thematic