So last year, the Rund um Frankfurt was actually cancelled due to a terrorist threat. This was rather different in circumstances, admittedly, but there is a precedent there. The reason for the cancellation was that police had raided the home of an individual known to have connections to terrorist organizations and who had been spotted scouting parts of the race route; the police couldn't guarantee the safety of the fans and participants and so the race was cancelled.
Here there's no known threat at time of writing that is directly to the race's fans and participants and the race doesn't pass close to Brussels and so they may feel that it's in order to continue with the race. With Frankfurt, although the same size race (.HC), it's the largest one-day race in Germany (even if Hamburg is WT) and it starts and finishes in a large city so policing the fans is more of a challenge, while Dwars doesn't; while a sporting event (especially an open road one like a cycling event) is a potential terrorist target, they may feel that Dwars is not as attractive a target as E3 or de Ronde and certainly given the current situation in Belgium it's quite likely that the number of people willing to travel to spectate will be limited too.
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