This piece from Scientific American explains why:
The 25,000 to 69,000 numbers that Trump cited do not represent counted flu deaths per year; they are estimates that the CDC produces by multiplying the number of flu death counts reported by various coefficients produced through complicated algorithms. These coefficients are based on assumptions of how many cases, hospitalizations, and deaths they believe went unreported. In the last six flu seasons, the CDC’s reported number of actual confirmed flu deaths—that is, counting flu deaths the way we are currently counting deaths from the coronavirus—has ranged from 3,448 to 15,620, which [is] far lower than the numbers commonly repeated by public officials and even public health experts.
. . . The question remains. Can we accurately compare the toll of the flu to the toll of the coronavirus pandemic?
To do this, we have to compare counted deaths to counted deaths, not counted deaths to wildly inflated statistical estimates. If we compare, for instance, the number of people who died in the United States from COVID-19 in the second full week of April to the number of people who died from influenza during the worst week of the past seven flu seasons (as reported to the CDC), we find that the novel coronavirus killed between 9.5 and 44 times more people than seasonal flu. In other words, the coronavirus is not anything like the flu: It is much, much worse.
I don't think we'll really know how many people COVID-19 killed until we see reliable numbers on excess deaths during the pandemic period. In any event, when the experts tell us that COVID-19 is way worse than the flu, maybe we should listen to them instead of the armchair epidemiologists and armchair virologists.. . . The question remains. Can we accurately compare the toll of the flu to the toll of the coronavirus pandemic?
To do this, we have to compare counted deaths to counted deaths, not counted deaths to wildly inflated statistical estimates. If we compare, for instance, the number of people who died in the United States from COVID-19 in the second full week of April to the number of people who died from influenza during the worst week of the past seven flu seasons (as reported to the CDC), we find that the novel coronavirus killed between 9.5 and 44 times more people than seasonal flu. In other words, the coronavirus is not anything like the flu: It is much, much worse.