There are many reasons. And we shouldn't take LD50 into account, that measurement is only useful for scientific research purposes, it just means that 50% of the population (probably lab-rats or other animal group) have died with such dose. In reality, animal research barely translate to humans, so there are many more variables and it's not as cut and dry as it sounds.
There's also the issue of purity, N is a drug that you can only get through veterinarians or the very few sellers that actually manufacture in powder, but mostly through A or one of them, you know what you are getting most of the time.
H is very different, this is a street drug and you can never be sure of it's purity, it may be a less potent batch or maybe cut with something.
I'm not saying that H is a bad method, I'm sure that it works best for some people since they are very different drugs, but at the end of the day, there is far more research and information available for N as a suicide method, we know about doses, stats and preparations. H is mostly "take as much as you have and hope for the best" approach.