Lord of the Rings isn’t just the story of Frodo. It’s the tale of the last great war between the forces of Sauron, and the beleaguered free nations of the West. The story is meant to encompass the full sweep of the war, not just Frodo’s journey into Mordor. The actions of all the members of the Fellowship are crucial to the defeat of Sauron. By resisting him, the armies of the West are buying him more time to complete his quest. At the same time, the escalating warfare demonstrates that Frodo doesn’t have time to waste: he must destroy the Ring before Sauron’s conquest is complete.
In this movie, the two main forces in conflict are the armies of Saruman, a corrupted wizard who now serves Sauron, and Rohan, one of the free nations of the West, under the command of King Theoden. The Rohirrim are one of the major military powers of the West, but they are caught unprepared by Saruman’s attack, and are forced to retreat to their stronghold in Helm’s Deep. No nation in the West can hope to stand up against the forces of Sauron and Saruman by themselves. If Rohan is destroyed, then the other nations, such as Gondor (of whom you will see more in the third movie) will be left with no strong allies with which to resist Sauron. Thus, the outcome of the battle at Helm’s Deep will be central to determining the outcome of the war.
Because Sauron is pretty much Hitler. He wants to rule the world for the sake of ruling the world, and Rohan does not intend to submit peacefully. Thus, war.
If you recall, at the end of the first movie, Merry and Pippin were kidnapped by orcs. By luck, they manage to escape, and flee into the forest. There, they meet the walking, talking tree guy. That’s an ent. Ents are big on environmentalism. What they don’t know is that Saruman, who historically had been an ally of the ents, is now working for the other side, and is clear cutting the forest to make war machines by which he intends to conquer Rohan. Merry and Pippin must convince the ents that Saruman is a threat to the forest that the ents live in. This takes some doing, but eventually they stir the ents to action, and they attack Saruman’s fortress, Isengard. (Isengard is one of the two towers mentioned in the movie’s title - the other is Sauron’s fortress in Mordor.) Saruman’s army is away besieging the Rohirrim at Helm’s Deep, so he’s pretty much caught with his pants down, and gets his ass kicked by the tree people.
As explained above, the other two hobbits aren’t actually being attacked: rather, they manage to raise another army (of ents) to attack Saruman and take pressure off the Rohirrim. By the end of the movie, Rohan has been saved and stands ready to aid Gondor, which is the other major power in the West, against the armies of Sauron. In the process, they also manage to neutralize Saruman, who is one of Sauron’s strongest allies.
Because Lord of the Rings is meant to be an epic tale, but Frodo’s struggle is largely personal. His quest is a small (but, ultimately, crucial) part of a world wide struggle, and the trilogy is meant to cover almost the entirety of that war (there’s a lot of other stuff going on elsewhere that we never see directly, because none of the Fellowship was directly involved). If Lord of the Rings were a trilogy about World War II, Frodo would be one of the scientists working on the A bomb at Los Alamos. Certainly, a crucial part of the struggle, but you’d also want to cover Normandy, and Iwo Jima, and other key parts of the war.
Also, because the stuff with Frodo is kind of tedious after a while. I can only watch so much footage of short people lost in a swamp before I want to see some decapitations.