Not unlikely? Can you point us to all the other times this has happened? You must know of several given your knowledge of how obviously likely this is.
This is a public bridge 5 years in the making and who knows how long in planning, and people have been extremely lucky that it collapsed at 3am and not in the middle of the day. It doesn't matter at all how unlikely this is in a given time frame if the impact if it happens at any point in time is catastrophic.