Why does Nnaemeka refuse to write a letter to his father in "Marriage Is a Private Affair"?

At the
beginning of the story, Nnaemeka is talking with his fianc©e, Nene, about his father. Nene
suggests that Nnaemeka should write a letter to his father to inform him of their engagement.
Nnaemeka refuses to write the letter, reasoning that:

It
would not be wise to break the news to him by writing. A letter will bring it upon him with a
shock.

Nnaemeka predicts that the engagement will be such
a shock to his father because Nene is not from the same tribe as them. His father will be
shocked that his son is marrying somebody from a different tribe and thus object to the
marriage. His father's objection will be especially robust because he has already found a
suitable wife for his son from the same tribe.

Nnaemeka hopes that if he
breaks the news to his father in person, then the shock and the objection will be less severe.
Unfortunately, Nnaemeka's hopes in this regard come to nothing. When his father hears that
Nnaemeka intends to marry a woman from a different tribe, he "applie[s] all possible ways
of dissuasion," and, these ways failing, eventually disowns his son.

Ironically, it is a letter from Nene at the end of the story which makes the father
regret or at least feel remorse for his actions. When Nnaemeka's father reads that he has two
grandchildren, he begins to feel "the resolution [that] he...built up over so many years
falling in." Later that night he imagines his grandchildren "standing, sad and
forsaken...shut out from his house," and, full of remorse, and with "a vague fear that
he might die without making it up to them," he endures a sleepless
night.