Needles cross a line for most druggies. Until that point, you can live in denial and say I'm not THAT bad. That's why hilbilly heroin (oxys) was a big deal, since it allowed people to chew and get ALMOST the same high, and still think you are OK
So it really means FAR less people take the 1st hit.
Also, it takes far longer to die on crack, it is far more social (a lot easier to share a pipe than a needle!), which means it grows via network effect. Heroin is a 1 on 1 type of thing when being shared. You have to trust the person more, since you may nod off and they could rob/kill you. Far less chance of that happening on crack.
So, when talking about individual affect, heroin is more likely to kill you in the short term, but crack will kill far more people.