thaluikhain said:
The indexes and supplemental material to LotR, not to mention all the other volumes written by Tolkien or his son on the subject, show quite clearly that the rest of the world was not simply sitting its on collective butt.
After Sauron's defeat at the end of the second age, the rest of the threats he had created didn't disappear. There was decades of cleanup and rebuilding to be done. There were still orcs and trolls running loose, and the Nazgul were still out there.
Once the world seemed secure, what could they do but wait for Sauron to resurface and deal with him then? No one could find him until Gandalf unmasked the Necromancer. Meanwhile, the world of men had repeated invasions from the Haradrim and the Easterlings to contend with. The wildmen were a problem that never went away, and more than once Rohan and Gondor were close to being overrun, not by the pure evil forces of darkness, but just by other ordinary men.
The northern kingdom of Arnor, after losing touch with Gondor in the south, suddenly found itself besieged by legions of orcs out of Angmar in the Misty Mountains. It turns out that Angmar itself had been founded by the chief of the Nazgul-in-exile, and the war that finally destroyed Angmar and chased the Witch King out did in Arnor itself as well.
So the world of men was in steady decline by the time Bilbo comes into the picture. The dwarves had been basically digging themselves deeper adn deeper into the earth in search of riches, until they encountered the breeding grounds of the orcs and goblins deep underground, and ended up in endless wars in the mountains that destroyed everything they had.
Meanwhile, the elves, the leaders of whom had lived in this world for centuries or even millennia, were growing tired. Understand, here, that the nature of the Eldar is stasis. They were immortal, with endless lifespans, and were raised in the Undying Lands of the West. They only came back to Middle-Earth because of Morgoth and what he took from them. The elves are not too concerned with the state of the world; all they want is peace and stability, so they closed themselves off from the meaningless concerns of men and dwarves and strove for their idea of Eden: stagnation.
So when Sauron finally did regain enough of his power to become a threat again, he had taken just long enough to find a world on the down-slope, weaker and more divided than he had ever seen it, and ripe for an evil invasion. It's no wonder that people like Denethor had lost all hope, to the point that they were willing to simply roll over and die, and that Saruman had decided the only way to survive was to join the enemy.