Why do you think Achebe chooses to create a protagonist for Things Fall Apart that hardly develops across the novel? How does this choice affect the presentation of the novel's key themes?

The simple and easy answer would be that great storytelling often relies on contrast. For example, coming-of-age novels tend to use very static settings to show that their protagonist follows the footsteps of many others as they grow. It helps to focus on the character and their internal conflict if the world they live in roughly stays the same. That is not a rule, of course, but it's one way to create the contrast. In this case, however, Achebe has chosen to show a society in the midst of a massive shift through the eyes of a man who does not change and does not want to change.
Okonkwo goes through life like a strong tree that would rather break than bend. Achebe manages to show both the positive and negative qualities that stem from that. On the one hand, Okonkwo's strength and stubbornness make him a man who is respected, feared, and praised. He suffers life's hardships without complaint and never gives up. On the other, his unwillingness to grow and question things leads him to accept things that he does not like. His terrible fear of being considered unmanly keeps him chained to cruel customs that might be in his power to change. Okonkwo himself understands all of this a little but refuses to consider it further.
His tragedy is therefore partly self-made and partly external. Achebe shows us how an uncompromising new power breaks a man who also refuses to meet it halfway. While Okonkwo himself does nothing to avoid his fate, the author also includes other possible options in the form of his friends and family. Okonkwo's son Nwoye accepts the changes gladly, but the price he pays is to be cut from the society he was born in. Okonkwo's best friend, Obierika, does his best to accomodate the Christians and to understand them, but it leaves him heartbroken to see that their forgiveness does not extend to those who do not bend to their rules.
Achebe chose Okonkwo as his protagonist to show the change through the eyes of the kind of a person who it hit the hardest. Okonkwo's point of view was the harshest and therefore ultimately doomed, which allows the reader to see Africa's pain from a less explored angle.

Okonkwo, the protagonist of Things Fall Apart, is a static character, as he does not change over the course of the book. However, his world changes immeasurably as the novel proceeds. Achebe creates Okonkwo as an unchanging character to show that even if Okonkwo doesn't want to change, the world changes so radically when European colonists and religious figures arrive that change is forced upon him. Okonkwo's static nature allows the author to highlight the ways in which Europeans bring inexorable change to Okonkwo's rural village, one of the themes of Achebe's novel.
Okonkwo is a symbol for tradition and the old ways, but the old ways no longer work in a world that is now challenged by Christianity and European forms of government and law. In the end, Okonkwo kills himself in part because he knows the old ways of life are no longer possible, and he is left with no way forward in a world that doesn't follow the old ways any longer.

The power of Okonkwo's story in Things Fall Apart is the relatively static nature of his daily life. He is very comfortably entrenched in the social order established, and questions little of it, even when he is made to feel sad or conflicted (as when he is forced to watch Ikemefuna's murder). The reason Achebe creates an essentially immovable character is so we can feel the monumental shift the story takes when the colonizers appear. When the outsider is introduced to an already functioning society, Okonkwo finds himself in a situation that leads him to despair. This despair motivates him to act in a way that is a 180-degree turn from the man he has been.
Okonkwo's suicide and the shift in point of view at the end of the novel reinforce the theme of social structures falling apart. Moreover, Ibo society has been corrupted by an outside influence, with no motivation or input from those who have built the society. This represents the loss of personal agency endemic to postcolonial literature.