Why does Okonkwo say that Ezinma should have been a boy?

Okonkwo says to himself several times: "She should have been a boy."

Later he says to Obierika: "If Ezinma had been a boy I would have been happier."

Later Okonkwo thinks "I wish she had were a boy." Ezinma is one of the few people who understand Okonkwo as well as she does. "He never stopped regretting that Ezinma was a girl."