Just a guess from an avid student/lover of history, but I'd guess it has to do with a lot of factors. As pointed out in the deleted comment, there isn't as great of a difference as it seems. The modern period has had its share of genocides, atrocities and injustices after all. Compressed into a historical perspective, it can seem harsh: massacres make the records, alms usually don't.

I'd posit that the rest of the change has to do with a combination the relative difficulty of survival increasing (for example, the percentage of the world population in danger of starving is much lower than in almost any other historical period) as well as an increase in how much we know about different cultures/people now. It's much easier to sympathize with someone when you can hear their stories first hand (through the media) and understand why they do something or live a certain way.