Maybe one virus kills another.
Or maybe the flue tests are being calibrated not to pick up flu just like the amplification cycles are increased to pick up minute traces of coronasvirus.
Probably the latter.
My understanding of how bacteria and viruses work is hat they create a favorable environment for themselves which turns out to be unfavorable for another bug.
They also monopolize the food supply.
Both those things together makes it much more difficult for another strain to get established.