Although Akeno is the goddess of thunder, Rias is the next head of the house of Gremory, a much more vital role with the devils. She also fights to break out of an engagement with Raiser Phoenix, a fight in which Akeno is eliminated by Raiser's queen before the battle with Raiser begins.
Akeno may have been taken out of the fight early, but Rias wasn't there throughout the majority of the fight. Not only this, Akeno fought on her OWN, where as Rias needed both Asia and Issei to protect her. Rias is also shown to be the main character straight away, and gives no real reason to make her a likeable character, whereas Akeno is introduced later and her bond with both Rias and Issei makes her more likeable. Another point might be that Rias is the next head of her household, but this doesn't really affect the way the character comes across to the audience, but Akeno comes across via her personality and bonds with the other characters.