Perhaps because people who are addicted to nicotine are likely to also have addictive personalities, whereas anyone can easily get addicted to heroin.

Kind of like how in WWII the British improved the survivability of their bombers by increasing the armor in the places returning aircraft weren't damaged, theorizing correctly that those were the places the planes that didn't return were hit.

If a person with an addictive personality gets on heroin, they're probably visiting the ER very few if any times before they hit the morgue.