Gramatical gender has very little to do with the words it describes. For example, in a particularly confusing twist: féminisme (feminism) is a masculine noun, wheras masculinité (masculinity) is feminine.

It's mostly due to the pronunciation and spelling of the word ending. Although in this case, the Académie Française decided feminine because COVID is an acronym, and the last word of that acronym is disease, and disease is a feminine word (la Maladie) because of how it is spelled.