The Kite Runner is a heavily male driven book; so its female characters, while believable, play minor roles. Hosseini might have chosen this tactic to demonstrate that societies such as Afghanistan's Golden Age or 1990s America, which are often heralded for their gender equity, in reality were not so equal. Consider Amir's mother Sofia. Even though she was a renowned literature professor during The Golden Age, her death almost completely eliminates her influence from Amir's life. Baba does not discuss her with Amir, and he does not appreciate the qualities she passed down to her son. Similarly, in the States, Soraya does not enjoy the independence or equity of other Americans or Afghan males.
Another possible motive for Hosseini's focus on male characters could be partially autobiographical. Many critics speculate that Hosseini's relationship with his own father might have been similar to Amir and Baba's. Baba wants Amir to become a doctor; Amir becomes a writer. Hosseini became a medical doctor first before writing his bestselling novel. The Kite Runner does not imply that women are insignificant; rather, it's primary focus is on male relationships (father/son, brothers, and friendships).
With his second novel, A Thousand Splendid Suns, Hosseini switches his focus to female relationships; so the male characters in it do not serve as narrators or receive the development that Mariam and Laila do. For both novels, one cannot accurately argue that the female or male characters are unconvincing; Hosseini simply chooses to focus on the plight of one gender at a time.