Because while HK was wealthy, it was not a "country", it never was, it was land leased by the Brits from China, then given a taste of democracy. The original agreement with UK was for China to slowly merge HK back to the mainland, what happened last year with HK, was China accelerating that process thus undermining the western rights that the HK residents come to know and love.
Taiwan is different since it was separated from the Mainland via Civil War. In fact most of the world viewed PRC as the renegade province in the early years after the war, and Taiwan as the true China. Once it was apparent that PRC was going to stay, (and they want access to the Chinese mainland), they started saying PRC was the real China and cut-off relations with Taiwan, but there are still nations today which has not defected and still considers Taiwan as the true China, and hence a sovereign state.