Why does Sora wake up in kingdom hearts 2 in the mansion

Because that was Namines' and DiZs' idea of a safe place to put him while he regained his memories, so Organization XIII couldn't reach him. Namine created the pod to keep him unconcious, so the process of getting his memories back would be undisturbed. He woke up in the 'Other' Twilight Town, not the digital, re-created one, made by Namine and DiZ to keep Roxas, Sora's Nobody, busy until the time was right for him to merge with Sora.