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%.
The reported confirmed counts have grown by a factor of ~1.5 every day, fitting reasonably well with exponential growth. If this rate continued, in 35 days it would reach the world population. Obviously, in the real world the growth flattens out way earlier, from natural causes, and especially because of the attempts to confine it.
Comparing the total number of cases or deaths to flu outbreaks from previous years does not make much sense at this point, when the numbers are growing rapidly, but in a month they probably are easier to compare. Neither does comparing average counts per week, because the growth is not linear. The data for the progression of flu (or any other disease) outbreaks exists, and comparing to their growth rate would put it better in perspective. Regardless of armchair analysis, the WHO declaration means it's something requiring unusual action, which flu is not.
Something else to consider is that flu outbreaks aren't typically taken very seriously by most people, while the widespread fear and media coverage of the 2019-nCov all but guarantees more serious responses from both the general public and the government.
Consider the recommendations to thoroughly wash one's hands during flu season as a precaution against getting the flu. How many people take that advice seriously? You can bet there'll be a lot more hand washing all around once this coronavirus hits people's local areas, not to mention all the mask wearing that'll happen (though unfortunately, most people will be wearing those dinky surgical masks which will be of dubious effectiveness), and people isolating themselves.
On the other hand, we have vaccines for the flu that are available well in advance of flu outbreaks (though not nearly enough people take them), while we've got no publicly available ones for 2019-nCov. That's another confounding variable that makes it hard to compare the two.
As an exercise in understanding exponential growth, for one month would you rather receive $20,000 or 1 penny on day one, 2 on day two, doubling each day until the end of the month. Try it with a calculator or program.
People are concerned because this has a high growth rate that, unless contained, would cause large number of deaths