Uh, how do you know it wouldn't have happened with a case? The case doesn't do much to tone down the impact (unless it's thick at rubber), rather it stops scratches. The iPhone has shown to be quite resilient against scratches, so a case is pretty much useless. This is the result of a heavy impact, something a case could not have protected it from.
Plus the case looks hideous, and give the iPhone an annoying feel in your hand.