Why are the suitors not prepared for Odysseus's return?

The suitors are not prepared for Odysseus's return for many reasons, beginning first with the amount of time Odysseus has been gone. It took Odysseus ten years to return home once he left the shores of Troy. All other Greek heroes of the Trojan War had either returned home by then or perished. In the suitors' eyes, Odysseus was never going to return home again (so there was no need to be prepared for his return).
Another reason the suitors are not prepared is because Odysseus did not announce his arrival once he lands on Ithaca. With the help of the Goddess Athena, Odysseus is disguised as an old man and is unrecognizable by all who once knew him, including his loyal servants, his wife and, most importantly, the suitors. This disguise allows Odysseus to gather information about the suitors without alerting them to his presence. Only his son, Telemachus, knows Odysseus's true identity.
Lastly, the suitors are arrogant and full of pride; they have been living in Odysseus's home for years and have not received true consequences, so they do not fear retribution for their actions.
Therefore, when Odysseus finally reveals himself, the suitors are not prepared for their ultimate fates.

There are many reasons why the suitors are not prepared for Odysseus's return. First, he has been gone a total of nearly twenty years, some ten years longer than the others who went off to fight in the Trojan War. All of the people who fought (and lived) returned to their homes a long, long time ago, so the suitors are fairly certain that Odysseus must be dead.
Further, when Odysseus returns to Ithaca, he is disguised as an old man, so neither his son nor his wife recognize him. Only his (very!) old dog seems to know who he is. If his own wife cannot identify him by sight, then certainly none of the suitors can. His disguise allows him to gather information in their presence without alerting or alarming them.
Finally, the suitors are incredibly arrogant. When Odysseus first raises his bow to shoot Antinous, "Death gave [the suitor's] heart no notice. For who could think that in this company of feasters one of the crowd, however strong, could bring upon him cruel death and dismal gloom?" Antinous, like many of the suitors, feels himself to be fairly untouchable and is, therefore, not expecting an attack. His sense of security, then, actually makes him vulnerable.