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?

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? 

I think Achebe wanted to show the cultural shift in his country by the colonial administration during that time period. Which could make him very distressed while writing, because I personally think when you are talking about your country’s history and culture and your country have been colonized once by another certain country, it is painful to talk about it as it is grey part of your own nation. I have the similar opinion, as I am Korean and Korea was colonized by Japan during WW2, I personally don’t want to talk about that time period because us Koreans got numerous violence by the Japanese government. However, I want to correct some of the assumptions that might have been made from foreigners who don’t specifically know about history of Korea, as I am Korean who supposed to know the “actual’ history of Korea, and correct other people if they know it in different ways- There are certain cultural misunderstandings such as Koreans can barely understand Japanese language as they are neighbouring countries and Korea have been colonised by them, but it is not true, as Korean and Japanese language is totally different, non of the grammar, alphabet, and structure of the language are the same. 

I felt like Achebe wanted to show similar ideas in Things Fall Apart. As many people don’t specifically know what culture or history of Nigeria, and from that people have wrong/untrue assumptions or stereotypes based on that. Chinua Achebe almost used half of the novel to express the atmosphere of the Nigerian tribe, to fully deliver the actual image of the Nigerian tribes who are not commonly accessible for other people who don’t relate closely.