Why does Achebe use juxtaposition with the details about the buried bicycle and the buried son?

Jonathan's buried bicycle is symbolic of his Igbo identity. Like his bicycle, he's had to bury that identity during the vicious civil war that's torn Nigeria apart for three long years. But with the war now over at long last he's able to dig up his bicycle—and with it, retrieve his buried identity—and move on with his life.
The bicycle's inevitably a bit rusty and has seen better days. But it's still in working order. And although Jonathan's son is no longer alive, what he represents—the Iwegbu family name—still lives on. For despite his sad loss, Jonathan has been able to keep his family together in the midst of all this chaos, bloodshed, and suffering. That explains his remarkable optimism in spite of everything that's happened. Just as Jonathan can use the bicycle once more as a way of providing for his family, he can point to the fact that most of his family are still alive as proof that he has much to be thankful for.