Because the field of view is narrowed and you can't hear your opponent, move your eyes more. Make an effort to scan the area. Along with the tunnel vision and auditory exclusion, the mind will shift into the sub-conscious mode. The sub-conscious operates much faster than the conscious, so your perception of time becomes altered. It seemed to happen in slow motion, didn't it.
Most fights also happen in slow motion.
That is due to a phenomenon called tachypsychia. There are numerous accounts of gunfight participants that recall seeing the front sight, squeezing the trigger, seeing the shot hit, and not seeing a reaction. The person thought that they had missed and fired again. They thought that the time between shots was a couple of seconds. Although it seems that the accounts are different, they really aren't.