There's no place like home. True if you live in Kansas; true if you live in a place where the trees come to life and badgers talk. Like the golden era from "Memory and the Past," Prince Caspian is all about reclaiming the idyllic home. The Telmarines have turned Narnia into a place incompatible for the Old Narnians, and the civil war is all about undoing that process so the Old Narnians can call their home, well, home. It's the ultimate extreme home makeover—only they have to tear down and rebuild an entire kingdom.
After Caspian flees from Miraz's castle, his story becomes one of rediscovering and preserving his new home.
The Pevensie children can't stay in Narnia like in The Lion because it's not their home anymore.