The reason for this is that when you hear about a tragedy affecting an individual, you often also learn about that person's life, maybe enough to spot similarities with your own. "That could happen to me!" you say, because everything has to be about you, and as a result, you care more. Maybe even enough to act, to help the person or prevent the same tragedy from happening to anyone else, especially if that anyone else is someone who looks like you. There's not a lot of conscious though in this; you see a person suffering, and without rationally processing anything, you care, and your brain later comes up with reasons you want to help.
But when you hear about a lot of people dying, you'll rarely learn enough details of their lives to connect with any of them as people. You might just hear the number of victims, or number of limbs missing, or some other statistic which is harder to forge an emotional connection with. That instinctive, emotional reaction to seeing another human suffer doesn't switch on, so your brain is just left with its plodding, rational toolkit. And for a big problem which looks insurmountable, the brain concludes nothing consequential can be done and sends you in search of the salty snacks which give you such comfort.
Crazily, that valuable empathetic reaction to seeing an individual suffer can apparently be shut down even by seeing a statistic. In one study, participants were divided up into three groups and asked to donate money to a specific cause. One group was presented with details on a specific starving kid, the second group was presented with statistics about a whole bunch of starving kids, and a third was told about the specific kid, but also got the statistical stuff about the bigger picture. People donated more to save the specific kid, as you'd expect. But the third group, the ones told about the kid and the bigger picture, gave less. Simply learning that more people were suffering made them care less. Other studies have suggested this effect occurs as soon as we're asked to consider more than one person in need. The human brain evidently has a powerful desire to help one individual life. More than one? "f**k em," says the brain. It's the reason we can lose a whole news day to the f*****g balloon boy, but still muster little more than shrugs when we talk about genocide in the Sudan or refugees in Bangladesh.