Finishing a war. There wasn't 70 years of peace. ROC port closure policy to blockade PRC lasted into the 80s, while ROC was used as staging for US operations into PRC throughout cold war. This is relatively recent memory, literally everyone in current and prospective PRC leadership was alive to understand the liability of TW as US asset, which has to be addressed eventually via conclusion of civil war. PRC hasn't even retaliated escalation to ROC measures of full island blockade and unrestrict infiltration over TW airspace. Just because PRC is behaving good (relatively) doesn't mean it will continue to. Nor that even if formally PRC had to start a new war to take TW, it will because the underlying security considerations are the same.