There's Such A Thing As Over-Analyzing....

Describing terrorism, Alan B. Krueger writes,

There's even a cold logic to the time of day that terrorists pick for their attacks, which also suggests a rhythm that's far from random. My analysis of U.S. government data from the National Counterterrorism Center reveals that terrorists are most likely to strike in the morning -- in time to enter the day's news cycle.

Leaving aside for the moment the fact that for years now we have had a 24-hour news cycle, perhaps there is a more obvious answer. When you are engaged in furtive, criminal acts, it is easier to avoid detection at night, and with daylight and daytime activity it becomes harder to either complete a crime or get away with it.