My theory is that people become addicted to meth/ice because they want to get high, but "good" drugs like marijuana and MDMA/mushrooms are not legal/available/cheap, so people buy/try meth/ice.
Put another way, my theory is that legalisation of "good" drugs like marijuana will lead to fewer people trying/becoming addicted to meth/ice. I wonder if its working out that way in places where marijuana is legalised.