If you got a good look at Kingdom Hearts III’s D23 reveal trailer you might have noticed that 4 other characters fought alongside Sora at one time in the Toy Story world.
When asked this by IGN, Tetsuya Nomura said that this decision was made due to the team deciding to remove the “Dimension Link” feature from Kingdom Hearts: Birth by Sleep. Stating..
“D-Link, that has been done away with. It was actually a compromise in a previous iteration because we were not able to display multiple characters on that platform in that game. We’re able to depict multiple characters. Up to five people can join, even more, multiple characters can join your party.”