It's a matter of precision and margin for error.
Bow you have 1 shot with an arrow to be placed in an area no bigger than a pie plate. Gun you could have 5 shots and get away with less than perfect shots and still kill the deer due to the impact and devestation of the slug. What if you could only have 1 shot with a slugun? What percentage of deer would be wounded? How many times when you're in the woods do you hear series of 2,3,4,or 5 shots as opposed to 1. I'd guess probably 80% are multiple shots.