Why does Achebe choose to bring in the European colonial presence only in the last third of the novel?

Achebe spends his time developing the culture of the Igbo people and the characters that inhabit it. Once character and context is established, the white people are brought in. This provides an interesting juxtaposition to the entrenched rigid beliefs of Okonkwo and the Igbo people. The arrival of the white man casts a light on Igbo culture at its best, its worst and finally on its eventual disintegration.