It's actually very simple. The Flu has a stable amount of people it kills per year and is already quite established in the human population so it infects a very large number of people per year. nCov on the other hand is a new and poorly understood virus and has only just been introduced to humanity. Extrapolating the number of people it's killed out a few weeks or months, the death rate would be much higher than the flu. The mortality rate is also not firmly understood, with estimates ranging between 2% and 20%.