Chased out by COVID or reduced by anti-COVID measures that have been in place for months such as masks, social distancing, staying home, etc.
That idea is out there, but I don't think it holds up to scrutiny. You might expect a modest, perhaps even significant, reduction in flu activity due to those measures, but we are talking a 99+% reduction in flu activity, and the same occurred in countries (and states) with much less restrictions than others such as Sweden, parts of eastern Europe, etc. So far the only places we are seeing any decent amount of flu is in SE Asia, where COVID is much less prevalent.