It is easier to deal with the other issues sans a hail of bullets.
"Love the people for what they are,not what you would have them be";old advice but still relevant.
Well, to be completely cynical, black markets thrive in the sectors of society that most voters don't go to. Whereas legalization brings more of the problems out to their neighborhoods. Hence the politics of the drug war.
But at the end of the day, legalization both would increase both the depth and the breadth of the overall problem, hence why even many inner-city politicians, whose districts bear the brunt of the black market, are opposed to legalization.