Because, as the article says, it's isolated and has a small population. Tasmania, in Australia, is not as wealthy as the mainland -- it's small and isolated. There are dozens of similar examples. Meanwhile, Singapore is small, but sits athwart what has been, for hundreds of years, one of the busiest trading lanes on the planet.
You might want to revise that sentence. Singapore was nothing 100 years ago.
Wikipedia: Singapore ceased to be part of the British Empire when it merged with Malaysia in 1963. Singapore lost its hinterland and was no longer the administrative or economic capital of the Malay Peninsula. The processing in Singapore of raw materials extracted in the Peninsula was drastically reduced due to the absence of a common market between Singapore and the Peninsular states.[5]
Since Singapore's full independence in 1965, it has had to compete with other ports in the region to attract shipping and trade at its port. It has done so by developing an export-oriented economy based on value-added manufacturing. It obtains raw or partially manufactured products from regional and global markets and exports value-added products back to these markets through market access agreements such as World Trade Organization directives and free trade agreements.[5]
By the 1980s, maritime trading activity had ceased in the vicinity of the Singapore River except in the form of passenger transport, as other terminals and harbours took over this role. Keppel Harbour is now home to three container terminals. Other terminals were built in Jurong and Pasir Panjang as well as in Sembawang in the north. Today, the port operations in Singapore are handled by two players: PSA International (formerly the Port of Singapore Authority) and Jurong Port, which collectively operate six container terminals and three general-purpose terminals around Singapore.
In the 1990s the Port became more well-known and overtook Yokohama, and eventually became the busiest port in terms of shipping tonnage