Ok, I'm gonna explain it to y'all...

The reason why they focus so heavily on demonizing marijuana is because there is such a heavy focus by us (by "us" I mean the pot smoking community) to get marijuana decriminalized. No one is advocating the legalization of crack or heroin, so there's no point in making anti-crack or anti-heroin commercials.

Do you get what I'm saying?

It's like a political ad campaign. If you want to win, you have to smear your opponent. If they keep telling people marijuana is dangerous, then people will keep voting not to legalize it.