But why heroin usage has raised?
I have read an (very convincing) argument that heroin usage is increasing due to the marijuana legalization. And no, it's not the typical "marijuana is a door the heavier drugs" thing, is merely supply and demand effect on the streets. Street dealers don't really sell weed as they used to now that better and legal weed exist, so they went on heroin. Heroin got a lot cheaper and widely available due to it (Unfortunately, I lost the article link).
I'm not arguing in favor or against legalization (I'm in favor all the way), my complain is how one folded the debate is. Numbers are great and they are definitely needed, but they may trick you into thinking that everything has a simple cause and effect when reality is much more complicated.
One striking example how lazy is legislation is the case of safer chemicals such as LSD or MDMA versus designer drugs such as 25i (nbomb). In some places such designer drugs are even legal (bath salts anyone?) due to how slow or careless the legislation is. As for LSD it's a long time in prison. The dealers are coerced to sell more dangerous substances.