I'm not sure I adequately understood your question, but I'm going to try.
Do I understand you're asking why scientific literature does not provide more support for this kind of legislation? It's probably because it doesn't exist. That's my best guess.
The studies have been done again and again and again, and while it's true there have been a handful of studies that appear to go the other way, in virtually every case there's something else that explains it.
For example, in the United States what happened is that they noticed in some cases that the crime rates were falling after the imposition of mandatory minimum sentences, but when they looked a little more carefully, they noticed that they had started to fall well before the mandatory minimums were adopted. Thus the level of violent crime was simply on the decline at that time, for a variety of reasons. It's virtually impossible to attribute that fall, though, to the existence of mandatory minimums.
So I think the best answer to your question is that it simply doesn't exist.