Question:

For the same reason that Canada, or Puerto Rico doesn't become an official state of the US. The problem with Hong Kong and Macau's current status is that even though they have self governing abilities they are still under the control of the PRC and the still have to follow any rules that they set for them. On top of that their independence comes with a expiration date, for Hong Kong it is 50 years from the time it was give over to the PRC government by the British and the Portuguese had a similar deal but I don't remember the time frame for the full integration into PRC China. As it stands now Taiwan is a completely independent sovereign nation that is in control of its own development with no limitations or foreign power dictating what it can do. Other than that Taiwan ROC is a democratic nation and PRC China is a communist dictatorship. Why would you give up governing control of your own country to another country?

Taiwan does not wish to be "STILL PART OF CHINA", because is NOT nor ever has been a part of the country you refer to as "China".

China and Taiwan already have deeply intertwined economic and social ties. In many ways they already operate as one country, despite their separate and sometimes acrimonious political relationship. Back when both places were poor they used to fight. Now there is too much money at stake and regular people on both sides want a permanent peace, so cooperation is the outcome.

Its because Taiwanese very well understand that the words of Communists doesn't count much. Even though China pledged to allow the administration of Hong Kong without any interference, they are already poking their nose and imposing their will over them.