I think you meant 'flu deaths are mostly caused by [bacterial] pneumonia'. One thought along those lines is that the spike in deaths we see is due to covid19 killing directly, death coming like clockwork X weeks after infection. Compared to the flu, which due to the requirement for a secondary bacterial pneumonia, has a higher variability in the timing of the death: at what time bacterial infection develops, which strain of bacteria, antibiotic interference, etc. Thus, we're going to see higher spikes for covid, and hopefully narrower.