One is to consider not causing ambiguity:

The period is a full-width symbol, which is easily confused with subscript 0.

For example, when the full-angle symbol "point A" is used, is it point A0 or point A? And then what? This is hard to distinguish. The use of "a" is not controversial.

Second, it is in line with international standards and follows the international common writing methods.

Thirdly, The Use of Punctuations in People's Republic of China (PRC) pointed out: "Small circles and small black dots can be used as periods, but small black dots are generally used in scientific and technological literature." The mathematics textbook itself is a science and technology book.