To answer this question, it helps to know a bit about the historical and literary context in which Achebe was writing. At the time Things Fall Apart was first published—1958—Nigeria was still technically under British rule, albeit transitioning to a state of independence. Achebe thus grew up under the shadow of colonialist ideology—in particular, its representations of Africa as a savage and unknowable Other to Europe's purportedly rational and civilized society.

This imperialist construction of Africa was especially evident in the literature Achebe studied while in college. Works like Joseph Conrad's Heart of Darkness and Joyce Cary's Mister Johnson might criticize imperialism, but not in ways Achebe could recognize or empathize with; Conrad leans heavily into the idea of Africa as a dangerous and mysterious quagmire, while Cary's novel focuses on a shallow and ignorant Nigerian man whose life is destroyed by his dreams of being an Englishman (charitably, one could argue that Cary is critiquing the hollowness of the imperialist promise to bring "civilization" to colonized regions, but he does so by relying on stereotypes of indigenous peoples as childlike and unsophisticated).

Achebe, then, was writing partially in reaction to these kinds of novels. Things Fall Apart is certainly critical of colonialism, but its critique is grounded in Achebe's depiction of Igbo society in the days before imperialism; throughout the novel, Achebe aims to show that Africa wasn't an empty or lawless region prior to colonialism but rather full of complex cultures that colonialism would later undermine. Even the novel's stylized dialogue is an attempt to capture something of both Igbo language and the social conventions it encodes.

Okonkwo himself, meanwhile, is a tragic hero in the classical tradition, which gives his fall a grandeur and seriousness that was missing from many Western portrayals of Africans: his death stems partly from forces beyond his control (including colonialism), but also from his own choices. In other words, Okonkwo is a full, morally complex human being in a way that a character like Mr. Johnson, at least to Achebe, is not.

Taken together, these factors are what make Things Fall Apart such a pivotal work in the history of postcolonial literature: the novel "writes back" to colonial depictions of Africa from a specifically African perspective.

Part of what motivated Achebe to write Things Fall Apart was the desire to capture the voice of indigenous African identity. Achebe was fascinated with living in Lagos, an area in which he was able to see the collision between old and new notions of African identity.  At the same time, Achebe recognized that the post- colonial condition of Africa demanded the emergence of new voice.  The traditional voice in African literature was driven by European visions of what Africa was.  The "African savage" and the notion that Africans were "uncivilized" in village life were aspects of what drove Achebe to configure something new.  The construction of a village identity in Africa which repudiates European construction of African identity is something that lingers in Achebe's mind as he takes in Lagos as well as the new condition of freedom that was a part of modern Africa.  These become the motivating forces behind why Achebe sought to write Things Fall Apart. Achebe understood that there needed to be a more indigenous voice to African literature.  He recognized that the way in which Africa was depicted through the eyes of a European had to be countered with an alternate vision of reality:  "Africa up as a foil to Europe, as a place of negations at once remote and vaguely familiar, in comparison with which Europe's own state of spiritual grace will be manifest." For Achebe, the need to depict a condition of being that was more "complex" and intricate helped to motivate his writing of Things Fall Apart.

It is with this in mind that Achebe conceives of the novel.  To avert the European construction of Africa as a realm of "negation" becomes one of the major factors in the novel's conception.  Things Fall Apart is where Achebe combines the traditional aspects of African identity into a setting in which the new is constructed.  The result is that a narrative emerges that is reflective of the indigenous African experience.  The desire to establish this is a motivating factor in Achebe's writing of Things Fall Apart.