And why is that?
Did they come from a region of China that has more microplastics in the water?
Are they mysteriously naturally a more evil people, because they live in a tiny island?
Are they evolving into different types of homo sapiens than mainlanders?
Or could it be once they separated from the mainland in 1949, a series of laws were passed which created a difference in the populace that didn't exist before?
What is the explanation other than a political one?
Culture. Taiwan has been bombarded with more a lot of Western media and modes of thought compared to the mainland. The general Taiwanese didn't become suddenly pro-marriage after it was legalized. Pretty much every poll on the issue that has been conducted from the 21st century on has shown that the pro-sodomy supporters/tolerator outnumber those who are against.
Also the KMT government that was running the show from 1949 on is actually pretty socially conservative so even if passing laws has the casual effect of changing people's mindsets (which I don't think it does as effectively as a change in culture/media), the series of laws that they passed wouldn't be the cause of the liberalization of Taiwanese society. What happened as Taiwanese culture become more progressive and socially liberal due to the effects of culture and mass-media via Western (mostly American) movies, music, sub-cultures/scenes and such, a traditional, conservative party such as KMT started to lose it's hold on Taiwanese politics and you started seeing these movements like the Sunflower Movement (basically the Taiwanese version of a lot of the Hippie stuff that happened in the 60s) push more socially progressive parties into power which then eventually resulted in policies such as the legalization of gay marriage. Summing it up, the Taiwanese population didn't become more liberal because a liberal government influenced them to be so. Rather, the Taiwanese population become more liberal due to influence from the usual suspects and they voted this sort of government into power. Going back to the American hippie refence I made a few sentences earlier, that would be another example of where culture was able to mass-brainwash a generation of citizens who then go on to push for laws to match the values that were planted into them via mass culture.
A big reason North Korea has been able to keep the sort of social values it has is because they close off their population from the sort of media and Hollywood influences that countries like South Korea and Taiwan get flooded with. I think that is way more important than whatever laws are on the books in this countries.
