Why does Hotaru have the best facial expressions?
Looking good is the most important thing.
What defines a good character is our ability to empathize with him/her. This is best when said character has a wide range of different reactions to different situations (conversely the stoic kuudere are difficult to relate to, which is why they are rarely main characters and when they are their character building often heavily relies on inner monologues). So in short: the best-er a girl, the wider her range of emotional expressions and the more reaction images she has. In reverse this basically means you can measure a girl's best-ness by how many reaction images she produces.
Case in point: Marcille from Dungeon Meshi. Almost every panel she's in is reaction image material.