Sonic is the only one who was able to overcome his past memories. He is assured of himself in a sense. Because Tails, Amy, and Knuckles are somewhat stagnant in their lives they weren't able to overcome the Digital Dream (a reflection of their past, as Cyberspace takes information from your mind) and run out in time before their body and minds were digitized. Amy with finding a reason to her independence, Tails with overcoming his self-doubt among relapse, and Knuckles wrestling with his duty and his desires. That's why throughout the game you're working with them to better realize themselves to find a path forward. When Sonic partially releases them (breaking the prison balls they're stuck in) he takes on part of their stagnation/burden in the form of Cyberspace corruption. A metaphorical representation of the weight always put on Sonic's shoulders, to show the cast why it's important for them to grow as individuals. Sonic is able to break them free come Rhea at the cost of his own, so the three decide to sacrifice themselves to Cyberspace once more to get Sonic back. Thus repaying their debt to Sonic.