Because there are a lot of disadvantages, too. Just look at the trouble in the modern EU--it's difficult to make the same currency work over very different economies. An economy that survives off of exports, just to use one example, has very different needs regarding inflation and the like then does an economy based on tourism or raw material exports. The only reason Mississippi and New York can share the same currency is because the federal government can tax the wealthier area and use the money to support the poorer areas to some limited extent. No colonial metropole would be willing to transfer money to support poor natives--that's the opposite of the point of colonialism. Having a currency for each colony allows you to tailor the economic explotation for the colonizer's benefit--whether you want to support agriculture or raw material exports, or industry (fairly rare), or what have you. A currency for each colony is just another lever of economic control, the same way that control over tariffs, taxation, and customs revenue are.
Ultimately, the reason you don't see one currency across the entire British Empire is because that is not the best way to enrich Britain at the expense of the colonies.
IIRC weren't most of the major currencies on the gold standard so there wasn't much fluctuation between them and the colonial ones pegged in some fashion to sterling anyway?
This is a good point. It's important to remember that governments and/or central banks at this time tended to have much less control over monetary policy than in the era of fiat currency. However, it's still possible to have a relatively tight or loose money policy. Just as one example, the Korean and Taiwanese Yen during the Japanese Empire were (mostly) convertable to Japanese Yen--which was backed by gold--not directly backed by gold themselves.
Since I brought up Korea, I think it's an interesting example of how money policy played into larger imperial goals. When Korea became a protectorate in 1905, the idea was that it would serve a very Mercantilist policy. Korea would serve as a captive market for the Japanese (just as India did for the British), absorbing Japanese manufactured goods, and exporting argicultural goods in return. Even though Korea was a member of the Empire (annexed in 1910), they still maintained a tariff wall between Japan and Korea.
However, during the First World War, the economic environment changed. European factories turned away from producing consumer goods, and dedicated themselves to military production instead. Overall demand increased as well. All of a sudden, Japanese domestic manufacturing was no longer enough to fufill the demand for Japanese manufactured goods. Industrialists wanted to expand manufacturing in Korea, as well, but the tariff wall made this difficult--Japanese machinary and investment could not freely flow into Korea. So, in 1919, the tariff wall came down. The Bank of Korea also adopted a new "easy money" policy.
The currency and the tariff wall were both parts of colonial policy.