-Bottom has eyelids

-Bottom has more attention given to the nose

-Bottom has lips and a hint of a chin

-Bottom has highlights and shadow in the hair

-Bottom has more proportionate eyebrows

It's cheaper to churn out slop that panders to otaku than to do any of the aforementioned above, and it's therefore more profitable especially considering who buys the merch. That really is all there is to it for why the above is far more common, especially now.