This is an excellent question to consider. To me, I think Achebe chooses to challenge received notions of history by giving us a fully developed picture of tribal life before the incursion of white men. We in the West seem to have two dominating ideas about colonialism: that the "colonised" somehow "needed" us because they were poor and ignorant, and secondly that we came in wisely bestowing our wisdom and aid like magnanimous gods, accepting the devotion and grateful thanks of our "subjects" with a royal wave.
This is perhaps a very stereotyped view, but this is certainly the view held by a certain generation of people such as my grandfather, who served in the British army in India during WWII. However, to counter such arrogance, Achebe presents us with a fully developed picture of a tribal society that doesn't actually "need" the help of the British. This tribal society has its own system of rules and regulations which actually works very well. Whilst we may consider them superstitious from our view point, tribal life "works" to maintain order and discipline.
Secondly, the missionaries, from their first arrival, are very unwise in the way in which they bring the gospel. Note how this is indicated from the very first by the mistake that the interpreter makes:
When they had all gathered, the white man began to speak to them. He spoke through an interpreter who was an Ibo man, though his dialect was different and harsh to the ears of Mbanta. Many people laughed at his dialect and the way he used words strangely. Instead of saying "myself" he said "my buttocks."
This somehow points towards the difficulty in "translating" Christianity into a different language and culture. It is the clash of these two incredibly different cultural belief systems that result in the "death" of Okonkwo, symbolising the death of Ibo tribal life. Thus Achebe seems to be greatly challenging received notions of history, suggesting that African culture was alive and well before the whites came and actually the whites caused more damage than they did good.