Mainland China's goal was to increase literacy by making characters easier to learn and use, but if that could be established, why didn't HK, MO and TW adopt them? Was it because they were against the communist government and its choices?
Mainland China was and still is administered by the government run by the Chinese Communist Party.
Hong Kong and Macau were British and Portuguese colonies, respectively, until the '90s after the British and Portuguese governments signed leases for them with the Imperial Qing Government. They were run by their own government structures entirely separate from that of mainland China (and relatively autonomous from the UK/Portugal too). Taiwan as I'm sure you know was and still is also governed by an entirely separate and democratic government system.
The simplified Chinese writing system was a pet project of the People's Republic of China for over a decade to make China's very complex writing system more accessible to the layman peasantry. After they developed it and implemented it across mainland China, they naturally couldn't impose this new writing system on regions that were British/Portuguese territory or on another country.