The one-party hold on China did not degrade but strengthened and gained ever more legitimacy because the concept of a one party state has cultural and civilization roots. This is an unbroken two millennia foundation of China. The "Communists" did not give up power, but they didn't need to. They [CCP - Chinese "Civilization" Party] just had to function as a continuously refining and practical imperial bureaucracy and governing organization, and they did a great job at it since the death of Mao.
Taiwan become multiparty because of artificial "de-Sinicization" processes started during Lee Tenghui's rise and continued after him.
Consider: the KMT lacked the spiritual characteristics that made the CCP so much more successful. The CCP enjoyed much greater support from the masses of Chinese people, and they shared in their minds a brighter common purpose, and they imagined and felt something new and better. It's the unity and drive the KMT lacked in comparison.
But the KMT failed that even on tiny Taiwan. The one party state failed to maintain itself on Taiwan.
I've been over this before. The PRC government stems from Western civilization.
Communism, the CCP, is firmly rooted in western political theory and philosophy, from Marx, Lenin, Trotsky and onto the first communist nations/revolutions - all thoroughly western.
China, under the western ideology of Communism, removed (as much as could be removed) Chinese civ basis during the cultural revolution, breaking the millennia spanning Chinese civ's/dynasties.