Japan was successful in brain washing people in Taiwan because they colonized them before Chinese nationalism even awakened.
Prior to the 20th century, only elites had a sense of collective identity - commoners identified only with their local villages and towns. They had no sense of being members of a larger Chinese nation, as there was no public education, literacy, or even a shared language among lower classes. For nationalism to emerge, those conditions must first be met, and they were not met in China.
It was the Japanese that brought those ingredients to Taiwan. They introduced public education, literacy, and a shared language - Japanese. For the local Taiwanese - the back bone of DPP supporters - this was their very first experience with nation-building, but it was Japanese nation-building, designed to convert the locals into loyal Imperial Japanese subjects, and it largely worked because there was no competing national identity to erase. It's a lot easier to fill an empty glass.
This is also why the KMT had to use such oppressive policies in order to erase this Japanese identity from the Taiwanese, and why they ultimately failed to replace it with a Chinese one. Having already been indoctrinated into their first national identity of being loyal Japanese, the cognitive dissonance of then being told that they're actually Chinese and were tricked into becoming Japanese, was too much for the mind to handle.
It caused a split in Taiwanese self-identity - a fraction accepted that they're really Chinese and embraced it, while others resisted it and either held onto being Japanese, or decided they were neither Chinese nor Japanese. Compare this to mainland Chinese, who for the last century or so had an almost exclusively Chinese national experience, and it's no wonder why Taiwanese ended up this way.