Before Odysseus can be alone, at long last, with his good lady wife, he has to reestablish his authority as king. When reading The Odyssey, it's always important to bear in mind that Odysseus is king of Ithaca, a role he takes very seriously indeed. The suitors who are paying court to Penelope and eating Odysseus out of house and home are not just unwanted house guests; they're impudently challenging Odysseus's kingly authority.
So, once safely back on Ithacan soil, Odysseus has to get his priorities straight. And though it may be tempting for him to dash off to the palace and into the loving arms of the wife he hasn't seen in twenty years, he knows that he must, first of all, do his duty as king. That means dealing with the suitors—not just slaughtering them, but purifying the royal palace of the spiritual pollution that they have generated all these years. It's absolutely imperative, then, for Odysseus to stay incognito before he's had a chance to settle accounts with the men who want to take his place as king of Ithaca.