Headcanon: Why Void Kong must be fought in DK Island...

Void realized he can't leave the island, so he decides to do what King K. Rool never could.

Destroy DK Island with his machine instead. However, he realizes his machine can't even do that, so he decides to just go after The Kong Family instead, crossing the line.

Or it could just be a fun little thing Nintendo just put in the DLC, what do I know?