Honestly, it's logic. You don't know how a game will do, so you don't go planning a sequel already. And the tone of later games is noticeably different. And if the secret ending foreshadows any game, it's mostly 358/2 Days It was a concept trailer for what they might do in the future.

Yes, there's a cliffhanger ending, but that doesn't mean they had all the plots planned out. They just wanted to make room for a sequel. In that way KH I was standalone.