Because Taiwan is, de facto, a separate country, but the mainland wants to keep up the pretence that it is entitled to do violence to it at any time if it so wishes.
It is the only a part of China that is not governed by the Communist Party of China (the People's Republic of China (PRC) because the Communists didn't manage to take over Taiwan and a handful of islands off of the mainland coast at the end of the Chinese Civil War. The effect was that the previous government of China, the Republic of China (ROC), "temporarily relocated" its capital from Nanking to Taipei, with the Kuomintang (Nationalist Party) putting Taiwan and other islands (Kinmen, Penghu and Matsu Islands and some others) under its control in a state of emergency for over forty years.
The effect was not dissimilar to that of the two Koreas, except the two Koreas are of roughly equal geographical size and at one time similar GDPs (indeed the Republic of Korea "south" was poorer than the Democratic People's Republic of Korea "north" until the late 1960s).
So as mainland China suffered under the jackboot of Chairman Mao for nearly thirty years, and lost tens of millions to executions and starvation, and stagnated, Taiwan was under the less brutal jackboot of Chiang Kai Shek for a similar period and became a rapidly developing industrial country. Of course since Mao died, China embraced corporatist capitalism and its economy has grown much like Taiwan did since the 1950s, but Taiwan also reformed although spent many years under tough authoritarian rule. In 1969 the first legislative elections were held to have representatives of Taiwan elected (all others were legislators who were elected before the end of the civil war representing Chinese provinces under "occupation" by the communists). As those legislators passed away they were not replaced so by the 1990s, the entire legislative assembly was subject to competitive elections. Martial law was ended in 1987 by Chiang Kai Shek's successor and son Chiang Ching-kuo, as Taiwan moved to being a fully fledged liberal democracy. By 1992, remaining mainland representatives were removed so the entire legislative assembly was subject to competitive elections.
Taiwan, as such, is a role model for China as an open, relatively free, liberal democracy, with rule of law, capitalism and political power tempered by the separation of powers. This, of course, is a complete anathema to the Chinese Communist Party and the People's Republic of China.
Of course, Taiwan (or rather the Republic of China in Taiwan - remember Made in R.O.C. labels on goods made a few decades ago?) has had an interesting history of international relations. With the exception of the UK (primarily out of concern of Hong Kong), most Western countries did not recognise the Communist takeover of China immediately. Australia and New Zealand did not until 1972, and even the United Nations seat for China was held by the Republic of China until 1971, as the United States vetoed the Chinese Communist regime (as it was called) from taking its seat despite strong efforts by friendly communist countries and developing countries lobbying on behalf of the PRC. Albania became Beijing's biggest champion. Both Beijing and Taipei regimes rejected each other being represented on the UN, but the US pushed for the PRC to be represented, not least because it seemed absurd that the world's most populous country (and a nuclear power) were not represented. So the PRC was admitted, and the ROC expelled. In 1979, the US switched recognition from the ROC to the PRC, but gave Taiwan a military security guarantee, as the PRC had always (and still does) reserve the right to reunify Taiwan by force.
Both Chinese regimes refused to have diplomatic relations with any country that recognised the other, but the ROC on Taiwan dropped this in the 1990s, as it had lost recognition and embassies with almost all countries as few could refuse the commercial, diplomatic and strategic benefits of having good relations with mainland China. Once Saudi Arabia (which had resisted the PRC because of its atheism) and South Africa (which shifted recognition not long after the end of apartheid) had dropped Taiwan, the ROC would only retain diplomatic relations with small Pacific Island, African or Latin American states, mostly through aid programmes (although this ended in 2000 as the PRC was able to outbid the ROC).
So Taiwan remains an oddity. In every de facto sense, it is an independent country, with a military, government and informal diplomatic relations with many countries. However, the PRC stops it from having formal diplomatic relations because it doesn't like the only part of China it didn't win in the civil war having a parallel status to it in international organisations or bilateral relations. It claims that to do so would imply there are "two Chinas", which of course there are as there are two Koreas, and were two Germanys and two Yemens (and even two Cypruses) regardless of claims of legitimacy.
So Donald Trump accepting a phone call from the democratically elected President of the ROC should not be a big deal, except for Beijing, it ruffles their sensitivities over reality. The US has a defence treaty with Taiwan, and should make it clear than any attack on Taiwan will be rebuffed by the US. The ROC on Taiwan shares the values of the United States and other Western democracies, the values of freedom, individual rights and government that is determined by the consent of the governed. The fact that this concerns the Chinese Communist Party is good, for it could do much worse than look at Taiwan and see an example of transition from one-party authoritarian rule to vibrant vigorous liberal democracy. Peace between Taiwan and the mainland is critical for both, so I think there is little real risk of any aggression from the PRC, but it should be clearer than that. A renewed US commitment to Taiwan because of its values.