Why would Achebe choose to set his novel prior to and during the arrival of the colonial administration as opposed to during the context of composition?

In 1958 Chinua Achebe published Things Fall Apart, two years before Nigerian Independence from the British. Chinua Achebe wrote Things Fall Apart, because he thought that the image that the European authors had made about Africa was primordial and totally obscene. More in depth Things Fall Apart destroys the stereotypical portrayal of Africans. This book was set in the 1890's and Achebe gives his perspective in detail on the colonization Nigeria by the British during the Scramble for Africa through the eyes of main character, Okonkwo who is an influential clan leader in Umuofia who strives to be the exact opposite of his lazy father Unoka. Achebe in general tries to portray both sides as equally as possible and without any bias. Personally I believe he decided to write his book during the colonization of Nigeria, in order to show an unbiased opinion about the situation that was occurring in his country, because it is known that Chinua Achebe was pro Independence of Nigeria and for that reason he decided to write Things Fall Apart in a difference time space. On the other hand, when Achebe was writing this novel he had adopted the Christian religion as well as being an Igbo. By knowing this the reader can see the full picture both from the side of the colonizers that are going in a hostile country with the goal of influencing them and the people of the colonized country, we see how they feel and how will their lives change from this situation.