My personal pet theory is that -- whether we admit it or not -- most people are somewhat... spiritual? Humans tend to see and believe in meaning where none exists.
Seeing great injustice like this is just really hard for us, because it's a constant reminder that either:
1. there is no meaning / purpose / higher-power / etc, or
2. we have been forsaken by whatever higher-power there is.
Both of these are uncomfortable, so it's often easier to just subconsciously fall into ignoring the issues and lulling oneself into a bit of happiness. Until something like this happens, then everyone has to act surprised for a bit; lest they admit to #1 or #2 above.