Taiwan will never join in, even if they wanted to. It would stir the pot too much with China.
But Japan's presence also means it hopes to gain aid for when China invades Japan. That may sound silly, but China claims a lot of Japanese islands as theirs. Some of which are very close to Taiwan.
They have these values tied to their identity, their ego. It's hard to change that. Hard to admit you were wrong if being wrong means having to become someone else.
This is also a problem on the left though.