Singapore is also an island, but its relevance to global trade has more to do with the strait it sits in than in being an island. Obviously there are more factors, compare Singapore to Peneng, for example.
For everyone else, Singapore is important because it sits right at the end of the historical trade route of the Straits of Malacca. There is a whole wikipedia entry about it.
Not a given, but an okay speculation fodder.
However, like many network efforts it's a winner-takes-almost-all market. Tanjung Pelepas is just across the border in Malaysia but it was too little, too late.
A lot of Singapore’s thing is also manufacturing from raw imports and then shipping out processed goods.
Hawaii isn't a hub because it's the only land for thousands of miles. You want the hub to be on the way to wherever you're going, not a quarter of the way across the world from there.
A lot of Australia's resources are bought and sold via Singapore precisely for the tax advantages of funneling things through there without the actual resources going through their ports.
In fact, there are. If you look at ports by tonnage, Port Klang in Malaysia is bigger than Rotterdam, and Tanjung Priok in Indonesia is bigger than New York.
So you can fill up a ship in LA with containers going to 10 different Asian ports, and drop them all off in Singapore.
This works best if everyone does that at one central location.
You're right other Asian ports are very big, but I think they more serve as origin or destination.