These mid air tragedies often happen in clear viz. Sometimes haze provides a uniform background which makes other aircraft more visible at close range, whereas in clear viz the background clutter can make visual acquisition of traffic more difficult. Any anyway, in any conditions the human eye is pretty poor at identifying traffic on a collision course (no relative movement) until the last second or so when it's usually too late to avoid.
And this time of year the sun is low in the sky, especially at the time this accident happened. If you're flying into such a sun.....
Usually "the sky is a big place and the chance of two aeroplanes being in the exactly the same place at exactly the same height at exactly the same time" keeps us safe from this sort of tragedy. But in areas of heavy traffic, sometime's it doesn't.