I remember he said on Facebook it's Toei who asked him to redraw the same art (they even sent him the images they wanted him to copy), give the characters thicker lines and more glossy coloring. It wasn't his choice. His personal art is much better than the official art he did for Sailor Moon. We don't know why Toei wanted that. Maybe they thought this way the art would look more modern and vivid? Maybe they wanted a uniform style guide they could use for international licensing?
I think the bigger question is why hasn’t Toei ever officially released these sketches before.