Because it was one particular case and the numbers have been looked at isolated. That and the fact that the first reports have been exagerated. Not to even speak about the way right wing groups used these reports...

I'll leave it to you to research crime numbers like rape, theft andn such for carneval in Cologne or the Oktoberfest in Munich. Because These are the numbers that provide the context.

Disclaimer: That does not in the slightest way imply that any of These cases are defendable, the least of all rape. Still, context helps to prevent knee-jerk reactions.