Cost (believe it or not) and effectiveness. It's so much easier to sweep it under the rug and "purge society" of it's problems than to fix it. But it goes so much deeper than that as well, the war on drugs, the industry of import/smuggling, etc... all that is pulled out into the open public forum and displayed as "you're so evil you want to close off access for immigrants".
Meth is gatewayed in by lesser things. As much as people don't want to accpet it, it's when they're at a party and things are going crazy, that's a gateway that a local dealer will pull it out and the peer pressure will push one into 'just a try'... it's so addictive that there is no going back, ever. They are constantly looking over their shoulder at the shadow of meth following them for the rest of their lives.