Because sometimes you shoot the man attacking you 9 times and he is still well enough to run. A law limited a person to a firearm that can only hold 10 or 7 (like New York) or 3 cartridges puts civilians at risk. What if this attacker returned? What if there was a second attacker? Or two dogs. Or the panicked victim misses half the shots?
Laws making "high capacity" magazines illegal don't stop crime, but they do put civilians at risk.