In addition to what Aidan said, it was mainly also to cover pre existing systems like in Ceylon/Sri Lanka - but also to satiate local trends.
Hong Kong/Straits Settlement/Malaya were decimal as many Chinese people in those places controlled commerce and since the 1700s had been using Mexican and American silver dollars as a unit. The system of dollar(Yuan) made up of 100 fen/sen (Cents) and the intermediate unit of Jiao (Ten cents or 1/10 of a Yuan/dollar).
Also the humble brass Chinese cash coin was set a 1/10 of a cent or mil. The Chinese had been issuing banknotes known as “Flying money” and used a base 10 system - most coins before the 1850s were single “cash”, but occasionally Tong Bao and larger coins were issued usually worth 10 or 100 cash and cash coins were usually carried in strings in units of 10 coins. The flying money notes were mostly 900 - 1350AD but were in units usually of 100s and even 1000s of cash. However by the 1850s, the banknotes were being reissued in the old Chinese style (Usually paper with illustrations of strings of cash coins) and then by 1890 in a Western style with Chinese script on one side and usually English on the other. Banknotes, silver coins in units of mace, candereens and silver taels (Worth about $1.30) were recent, but cash coins go back at least to 220BC. The coin was very low value and designed to be a base unit for everything.
Trade with America, Netherlands and France also precipitated a decimal system and this extended to SE Asia as well. In 1870 Japan also adopted a decimal system again of a main unit made up of 100 minor units and 1000 minute units (10 rin = 1 sen, 100 sen = 1 Yen). Korea followed not long after with a decimal won by 1895. By 1860 French Indochina was using a similar unit of 1 piastre made up of 100 centimes and again 1000 sapeques.
The British responded with decimal mils for Hong Kong (1/10 cent) and the coin looked like a Chinese cash coin with a hole, same with Indochinese sapeques. Hence why in 1862 The British made Hong Kong, Singapore and Malaya decimal and the same with Sabah and those places.
The West Indies was mixed as places like Jamaica, Bermuda, Bahamas and Cayman Islands remained sterling until the 1960s, but many of the Leeward colonies went decimal in 1950 with an Eastern Caribbean dollar, again reflecting the legacy of decimal French currency used in their colonies and the older heritage of Spanish dollars and “bits” which were single reals worth 12½ cents. Add to this the gumbo of decimal American currency and Latin American republics mostly adopting Spanish real/dollar based systems in the early 1800s and most having their own decimal “pesos” by the later 1800s. By 1950 the UK was late to the table, but set up a new decimal dollar was pegged to the pound at $4.80.
This made sense as $4.80 meant that each decimal cent was worth exactly a halfpenny and thus a shilling was 24 cents, and that is why you get stamps with values like 24c, 48c, 60c, $1.20 and so on as they transferred to 1/-, 2/-, 2/6, 5/- etc. Pegging it at $5 seemed silly and this was after the 1949 devaluation.
The only place this system came unstuck was the British Virgin Islands, which in 1962 adopted the American dollar to adjust to the currency of the US Virgin Islands (Previously the Danish East Indies - another decimal currency since 1863) and thus stamps were denominated in US currency which was worth more than the Caribbean dollar. Today the Eastern Caribbean dollar still holds sway, although its quite worthless being around 20 cents. The sterling Caribbean colonies all went decimal between 1962 (Trindad and Tobago) and 1969 (Jamaica, Cayman Islands).
East Africa adopted a decimal currency in 1907 when it was separated from the Indian currency system, although they adopted a decimal shilling made up of 100 cents and again this was a low value currency. This put it inline with decimal currencies used in French and Italian colonies. Many Arabian and Indian colonies kept a rupee system however except places like Mauritius and Seychelles, which had the rupees, but like Ceylon a decimalised one of 100 cents to a rupee.
Finally West Africa had a bizarre hybrid system which was 10 cents = 1 penny and 12 pence equal a shilling, as low value holed coins worth 1 cent, 5cents and 10 cents were issued through to the 1950s. These were 1/10, ½ and 1 penny as well. This included Sierra Leone, The Gambia, Nigeria and Gold Coast (Ghana) along with parts of British Cameroon (Now the English speaking part of Cameroun). Nigeria was the last place to abandon sterling currency in 1973.
I love coins. Especially silver, gold and anything really old.
Member of the Royal Numismatic Society of New Zealand and the Auckland Numismatic Society