“Why do you think that?”
Because a nuclear arsenal becomes completely worthless the second you use it, in the modern world. It only has value if you don’t use it, but everyone thinks you are willing to.
Having more nukes makes the US and Japan more unwilling to defend Taiwan, which in turn makes the CCP more willing to run the risk of doing so to obtain the internal political and emotional (for the leadership) benefits of ending what they see as western conquest of their country.
The more nukes they have, the more plausible an invasion followed by a threat to launch them becomes. If they threaten us with nukes because they have already attacked Taiwan, and the US and Japan support Taiwan, they will quite possibly launch nukes. Why? In part because they’re the CCP, one of the most sinister organizations in the world. Therefore, they can attack Taiwan, and possibly chase the US out of NE Asia.
That’s how I see it anyway.