if you have turbinates in your sinuses, and they inflame when sick/allergic, it can reach a tipping point where you can't smell any more. So one flu may inflam

if you have turbinates in your sinuses, and they inflame when sick/allergic, it can reach a tipping point where you can't smell any more. So one flu may inflame them more than another, or you have have grown more/larger turbinates later in life where you more easily lose your sense of smell. perhaps covid-19 cause the loss via some other mechanism entirely, I have no idea.