They use things like smiles and krokodil because the drugs they usually go after, ecstasy and heroin in this case, are unaffordable, or they can get more of the former two than they can of the latter two.
Why do people start using drugs in general? Some are addicted because they used certain kinds of drugs to treat medical conditions, like folks who suffer from chronic pain occasionally getting hooked on opiates. Others, well...they're poor, they're depressed, life to them is a crushing experience, and they feel that the only time they're happy is when they're high. Its an easy out for life's many problems...the only problem there is that drug addiction doesn't really help your problems, and ends up creating more than what you had to begin with.
Also, and I'm talking about America in this case, we don't have comprehensive drug education. Our drug ed is at pretty much the same miserably failing level as our sex ed, with "just say no" campaigns being similar in theory and in practice to abstinence-only sex ed. I'm not saying we should scare kids shitless with horrible, graphic descriptions and images of what drug addiction can do to you, but we should teach them what they do. No scare tactics, no bullshit, just straight, no-nonsense facts.
While it may not eliminate the problem, and honestly, nothing ever will, it'd probably go a fair way toward curbing the number of minors becoming addicts. Education works, and we really should do it more often with shit that matters.