Why did prosperous trading cities emerge in 19th C Malaya but not Sumatra?

The straits of Malacca are one of the choke points of world trade. As a result of them, a series of cities got rich and attracted large numbers of Indian and Chinese citizens to become multiethnic hubs: Malacca, Penang and, most famously, Singapore. But why were all these sorts of cities on the north side of the Straits? Why did none of the Sumatra cities become similar?
 
Malacca was controlled by the British, who had a larger empire, and more important, a shifting focus to free trade by individual merchants away from company monopolies Sumatra on the other hand was controlled by the Dutch who kept fast at their spice monopoly. Furthermore, the British created the Signaporean trade post in 1819, while the conquest of Sumatra by the Dutch happened later.
 
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