Why can't I reach Sevii Islands in LeafGreen after beating the game?

Question

I went to the port on 3 island and the cave is the same as it was

Answer 1

In order to access the rest of the islands after you beat the elite four, you need to go to the Pokemon center on One Island and get the Rainbow Pass from Celio. Additionally, you need to have the national dex in order to access them.

Source: experience