What makes Achebe's Things Fall Apart a tragedy is the destruction of a way of life and the suicide of the protagonist.
The Ibo tribe is shown in part one of the novel in its traditional form. But in parts two and three Christian missionaries and then the white man's government impose themselves on the tribe, eventually leading to the end of the traditional way of life shown in part one.
Okonkwo, the protagonist, returns after being exiled for seven years and finds his village in turmoil and vastly changed, due to the presence of the missionaries, etc. He is unable to adjust, and the tribe will not follow him in rebellion. He subsequently commits suicide.
The destruction of the Ibo tribe's way of life as well as that of Okonkwo is of course emblematic (serves as an emblem) of Africa's fate under colonialism.
Inspiring the title of Achebe's work, Yeats writes, "Things fall apart/ Mere anarchy is loosed upon the world." This might represent why there is a sense of tragedy in the book. It is not merely that there is a sadness associated with Okonkwo's narrative. The real tragic condition in the work is that there is a complete rupture of the order that helped to provide meaning and in its place is a sense of nothingness. The tragic condition that results is that human beings are placed in a predicament of having structure taken away and then not having anything to replace it, only a vast emptiness where the past has been reconfigured to be useless in a present that is foreign. Okonkwo's recognition of how colonialism has forever changed the village and its values helps to highlight this tragic condition where individuals are pitted between equally undesirable ends, only to find unhappiness wherever they turn.