Taken from the article:

One of the agreed criteria includes “reciprocity,” meaning that the EU won’t allow incoming travelers from countries which have barred European citizens from entering. Another condition when deciding whom to bar is the rate of new Covid-19 cases, according to another internal document seen by Bloomberg

I assume that in Vietnam’s, Taiwan's, and possibly some other countries’ cases (since the draft list had a lot more than just 15, still no US, though), they simply weren’t comfortable enough to let EU travellers enter yet. Even China was only allowed to be on the list if Beijing agree that, and relationship between EU and China is much more important.