It is common for one respiratory infection to drive out others. https://www.thelancet.com/journals/lanmic/article/PIIS2666-5247(20)30114-2/fulltext discusses how in 2009 Rhinovirus infection kicked H1N1 infection to the kerb, through the “stimulation of antiviral defences in the airway mucosa”. So that’s one theory as to why it happened in the past, at any rate.

So, if the same thing happened here, when the flu viruses went to infect more people they found that the people were already fighting covid with their own mucus membranes, etc. Which meant that the flu viruses were out of luck. They'd been out-competed for access to people they can reproduce in.