In my experience it has nothing to do with evidence. People are emotional creatures and said emotion pervades every choice they make, whether they think themselves to be rational or not. Incidentally, the rational ones have the most trouble accepting their emotions which in the grand scheme of things makes them among the most ignorant of society.

I think that the changing attitudes are simply due to the old guard dying from old age and disease. When there are sufficiently few people who grew up in the era of hardcore anti-drug propaganda then there will be a policy change.