It has nothing to do with getting close to the BB barrier. The virus likes to infect respiratory cells, which are present in the nasal cavity. A saliva sample might contain viruses in an infected person, but only because it is in the right general area, not because the virus infects the actual salivary glands. Pure respiratory excretions/cells is the most likely location to find the virus if the person has it. Samples taken less deep into the nose are almost as good, but the sensitivity is a little bit worse(you might misdiagnose someone who has the virus). I am not sure how sensitive the test is for saliva samples, but I assume it would be lower.
