So to answer this one, you have to all the way back to the first Sino Japanese war. After a surprise Japanese victory, the then Qing dynasty government was forced to cede Taiwan to Japan. For 50 years from 1895 through to the end of World War 2, Taiwan was a Japanese dependency.

Following World War 2, the Nationalist government in 1947 found itself facing mass protests against corruption and incompetence. In classic Chiang Kaishek style the government responded to these protests brutally, leading to the “28 February incident”in which soldiers fired on demonstrators, killing up to 28,000. This incident was followed by decades of martial law, in which dissidents were persecuted or disappeared.

From the end of the civil war in 1949, Taiwan had a large influx of migrants from the mainland, initially Nationalist soldiers and their families, but also regular immigrants from the mainland and Hong Kong in the decades to follow.

So very broadly speaking, you see the formation of two schools of thought within Taiwan. Firstly you have what are called the ‘Green’ Taiwanese. These are the Taiwanese who lived in Taiwan before the civil war and feel no special connection to the mainland. I should mention they also include members of Taiwan’s 13 aboriginal peoples. They do not advocate a reunion between Taiwan and the mainland, instead for an independent Taiwan and an independent Taiwanese identity. They are represented by a coalition of parties lead by the Democratic Progressive Party.

In contrast, you have the ‘Blue’ Taiwanese. Those that came to Taiwan after the civil war, believe that Taiwan is fundamentally Chinese and that Taiwan and China should reunite, albeit under a democratic government. They are represented by the Nationalists (KMT).

The DPP only first won a presidential election in the year 2000. By that time, the KMT in Taiwan and the CCP in the People’s Republic of China has made progress in thawing diplomatic relations, in no small part by each government reaffirming the eventual need for reunification and intentional ambiguity on issues of sovereignty. The DPP candidate who won the election, Chen Shuibian, was well-known for his long standing pro-independence stance. From the CCP perspective, the assertion of Taiwan’s independence from mainland China represented a fundamental step backwards in diplomatic relations (as far as CCP goals were concerned). Obviously it’s hard to discuss this trend further as I fall within the 20 year rule.

So you can see how the KMT might be seen as the ‘pro mainland’ party on two fronts. Firstly, they consider that Taiwan ought to be under one government with Mainland China (as opposed to the other major party, the DPP, who would rather see an independent Taiwan). Secondly, their stance on an eventual reunification with China puts them in a better position to have cordial relations with the CCP, who technically have the same goal.