Why are Macau and Hong Kong still using their own currencies even though they are now under China rule?

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Hong Kong was a crown colony of the United Kingdom from 1842 until the transfer of sovereignty to the People's Republic of China in 1997. The Sino-British Joint Declaration and the Basic Law of Hong Kong stipulate that Hong Kong operate with a high degree of autonomy until at least 2047, fifty years after the transfer. Under the policy of "one country, two systems", the Central People's Government is responsible for the territory's defence and foreign affairs, while Hong Kong maintains its own legal system, police force, monetary system, customs policy, immigration policy, and delegates to international organisations and events. Jul 11 07, 5:34 PM

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The status of Macau since reverting to People's Republic of China sovereignty on 20 December 1999 is defined in the Sino-Portuguese Joint Declaration and the Basic Law, Macau's constitution promulgated by China's National People's Congress in 1993. The Joint Declaration and the Basic Law specify that Macau's social and economic system, lifestyle, rights, and freedoms are to remain unchanged for at least 50 years. Under the principle of "one country, two systems" articulated in the Basic Law of Macau,[13] Macau enjoys a high degree of autonomy except in defense and foreign affairs. Macau officials, rather than PRC officials, run Macau through the exercise of separate executive, legislative, and judicial powers. Macau maintains its own separate currency, customs territory, immigration and border controls, and police force. Jul 11 07, 5:37 PM