There are a bunch of reasons for how the difference plays out it as it does, and you could go into target audiences and cultural perceptions and yadda yadda.
But the main one, I think, is that both aesthetics originated as a sort of rebellion. In postwar Japan, there was a strong expectation that adults should be sober and sensible and serious; hanging onto willfully childish things like cute characters became a non-confrontational way of pushing back against these expectations. In postwar America, adulthood and conformity was not associated so heavily with seriousness, but the adult world was expected to be neat and picturesque and nice, so grotesque and deformed characters similarly became a way to express dissatisfaction with grown-up norms.