Why is Things Fall Apart a post-colonial novel?

One purpose of post-colonial literature is a consistent correction of the false perception that colonized people had no true culture of their own or were uncivilized before the arrival of the colonial power.
Achebe’s Things Fall Apart goes to great lengths to demonstrate that the Ibo have a rich heritage of cultural traditions and a fully-developed civilization that includes courts of law, formal religion, and systems of diplomacy and trade. When the English arrive, they do not bring a previously-missing light of civility with them. By detailing the highly-developed cultural institutions of the Ibo, Things Fall Apart calls into question existing stereotypes regarding the peoples of the African continent as uncivilized. 
A substantial portion of the novel is dedicated to depicting these aspects of Ibo civilization and culture and this fact alone is enough to qualify the novel as a work of post-colonial literature. This is especially true considering Achebe’s expressions elsewhere about his intentions with the novel and his feeling that Nigerians and Africans in general had been too often shown as uncivilized savages in Western literature. Far from being uncivilized, the people of the nine villages in Things Fall Apart possess the same civil and religious institutions that the English claim to bring to them to save the Ibo from a brutish or uncivilized state.
The English presumption that the Ibo are without civilized principles (lacking religion and law) is at the heart of the novel’s conflict, as the native people who rely on the integrity of their culture struggle to maintain their identity when a powerful and determined colonizing culture attempts to uproot and replace that identity.
While individuals like Okonkwo’s son Nwoye are served in positive ways by the colonizing cultural norms, many people like Okonkwo are deeply compromised and, in the case of Okonkwo himself, powerfully damaged by the loss of cultural identity.

He saw himself and his fathers crowding round their ancestral shrine waiting in vain for worship and sacrifice and finding nothing but ashes of bygone days, and his children all the while praying to the white man’s god.

Thus the novel articulates a response to the presumption that Nigerians like the Ibo had no civilization or culture prior to the arrival of the English colonists. In doing so, it suggests the questionable ethics of the colonial mindset. If the colonists’ rationale for occupying and dominating this region is based on the notion that the Ibo need to be saved from a lack of law and order and a lack of religious principle, Things Fall Apart presents an idea that fundamentally undercuts the basic validity of that rationale and in doing so posits a post-colonial critique of the colonial mindset.