I think its because she's more plus sized, like ACTUALLY plus sized. She's not appealing because of her tummy/chubbiness. People will tell you its because she's not popular, however there's avatars and avatar bases that are WAY less popular than Ichigo with way more assets made for them. A lot of if not MOST of these avatars and assets are made by Asians, and in that part of the world, chubbiness/plus size/ anything over 120lbs is considered obese to their standards, which means a lot of asset makers don't find her attractive because of her body type. Sure there's a few exceptions, and we have as many assets for her now BECAUSE of those few exceptions. But exceptions don't replace the norm that through out the entire world, NOT just Asian countries, skinny with big booba and no booba will always be the the preferred look over someone with Ichigo's body type