Why can oral swabs not be done instead of the nasal testing for all?

Oral swabs are less favourable due to their lower sensitivity in comparison to nasal or deep nasal specimen collection. Hence, false negative rates would be considerably higher. As a corollary, if oral swabs were used for all test subjects, there would be a far higher number of true positive COVID-19 cases compared to test-confirmed positive cases. This obviously has numerous adverse consequences – most notably of which being infected, infectious individuals unknowingly spreading the disease due to falsely negative test results. For more information on COVID-19 testing, please refer to Public Health Ontario’s COVID-19 Laboratory Testing Q&A’s document, seen here