Why are “nasopharyngeal swab”, “saliva” as well as “nasopharyngeal and oropharyngeal swab” the only specified specimens for testing?

The specimens required for the certificate at the time of entry into Japan are the same as the
specimens recommended for testing asymptomatic persons in Japan. The recommended
specimens for testing asymptomatic persons are listed in the Guidelines for Testing for
Pathogens of Novel Coronavirus Infections (https://www.mhlw.go.jp/content/000747986.pdf),
and currently include “nasopharyngeal swabs” and “saliva”.
In addition, “nasopharyngeal and oropharyngeal swab” has been added to the list of valid
specimens for travelers arriving in Japan after midnight (Japan time) on July 1, 2021, following
an approval by the Council of the MHLW on June 25, 2021, as a valid specimen for predeparture testing of travelers to Japan.

Information obtained from:  Ministry of Foreign Affairs of Japan Website