The problem is that to most Republican voters, the fact that this didn't actually impact drug supply isn't really all that important. What matters to them is that - theoretically - "bad people" got killed. They're more interested in punishing people they see as evil than they are in actually improving the world. Even the ones who won't openly admit - or even admit to themselves - that they just like seeing brown people get killed will still defend this as an ultimately good action because it satisfies their twisted notion of justice. The meting out of punishment is what matters to them, far more than whatever social good that punishment is actually supposed to achieve.

It's the same reason why they have so little interest in welfare policies even when those policies are demonstrably cheaper than the alternative. They won't give shelter and money to homeless people even when it's shown to reduce crime, for example, because the most important thing to them is that no one should ever be given something that they "don't deserve." They're less interested in reducing crime than they are in seeing the poor suffer for whatever imagined sins made them poor.