Ships larger than 300 metres cannot use the dock right in Cairns, also larger ships cannot moor very close to the township due to the tides and being to shallow. They have a channel which has been dug out and constantly dredged which would allow it closer but then the ship would block the channel for all other ships. Tendering would take much longer than 30 minutes and the ship is as well protected from weather changes.
Yorkers know is considered a better option due to being a more sheltered area for tendering.
All I know is that it is a real pain of a port when you are on a larger ship.