One of Odysseuss main incentives for returning home is that he wishes to be reunited with his father, Laertes. Odysseus learns to appreciate the value of home and family even more during his long journey. When speaking of his desire to go back to Ithaca while he is still in Phaiakia, Odysseus asks, Where shall a man find sweetness to surpass his own home and his parents? (book 9 line 34-36).
If the book ends before Odysseus has reveals himself to his father, then Odysseuss goal of seeing his family again has not been completely fulfilled. His father is consumed with grief about his missing son. Laertes is not taking care of himself at all; he is sick and emaciated from sleeping on the ground in the mud and not eating. In the hierarchical mindset I dont think you could leave the king (Laertes is still king until he dies) or a father in that condition and not face wrath from the gods.
The Greeks had tremendous amounts of respect for elder family members and it doesnt make sense that Odysseus, who is supposed to be extremely generous, would let his father suffer after he returned home- its too cruel. In terms of number of pages the journey home is not really that big or significant, while ousting the suitors and restoring right at home is huge. The focus of the book is on what Odysseus did once he got home to accomplish that goal- rather then how he got to Ithaca itself. Odysseus comes home, and murders the suitors, but that only resolves some of his problems.
He still has the families of the suitors to contend with, for there is a very good chance that the Akhaians will not accept Odysseus as king again after he has murdered a good number of the high-ranking men in town. Before the book ends, it is essential that Odysseus make peace with the families of the suitors and reunite with his father, Laertes. If the Odyssey were to end while Odysseuss power was still in question the main goal would not be accomplished, and the book would be incomplete.