They WILL go through most non-plate body armor, as there is much more force behind the point of that blade than there is a bullet. Also, knives have a much higher range of attack (they can be swung around) whereas a gun has to be aimed to be accurate. Also, many people can cover 7 yards in less than two seconds (that's 21 feet). That's a fairly large danger zone.

I'm not excusing the officers at all, but just pointing out that they may have been much too close to the suspect holding a knife in the first place. Maybe there just wasn't enough room.