The first and easiest to understand element of this is they have a very different fatality rate, the two conditions. The flu has approximately a 0.1% fatality rate, whereas the bottom estimates for the coronavirus are at about 1%. But some countries are showing significantly higher numbers than that. On just that level, it's at least 10 times deadlier for people who get infected. On top of that, you have a higher estimated transmission rate from coronavirus than from the flu. Each person who has the flu generally will give it to 1.3 people compared to the at least two people that are infected from each person who has the coronavirus. Each person who has the virus will, on average, give it to two people and thus triple the number of cases in each iteration.