Riku couldnt have stepped through the door and why would kairi go with sora, she couldnt help him find riku she'd just be in the way
I think the reason riku and mickey couldnt have came through the door is because he was on the dark side of the heart or something like that, I read it in an interview with tetsuya nomura, Same reason goes for mickey
HERE IS EXACTLY WHAT HE SAID IN THE INTERVIEW
Question: In the ending, Riku and the King are on the other side of the door. Why is that?
Nomura--As far as the King is concerned, he was on the other side from the beginning. He disappeared because he went to the world on the other side in order to find one of the two keys. There, he has his own adventure in the same way as Sora. As for Riku: his body stolen by Ansem, his heart did not remain in this world. The other side of the door is, just as it appears, darkness. So Riku's heart went to the world on the other side, that is to say just as Ansem was saying at the end, the world of darkness. Thus, when Ansem disappears and Riku returns to his original body, he is in the world of darkness.
Question: Well then, when Riku has returned to his own body, why doesn't he come back to the world on this side?
Nomura--That's another thing I've been asked by lots of people, but the answer is actually written in the World's Prison computer message. It is implied there that only darkness can pass through that door. Riku and the King are not creatures of darkness, so they can't come out through that door.