Minimum sentences does not work. Any one who deals with criminals on a daily basis will tell you that.
A career criminal, juvenile delinquent, enraged spouse, etc. does not care about minimum sentences. He/she will still carry and use a weapon during the act. And if he/she cant stab you with a knife, they'll use a screwdriver, pen, bottle or whatever else is available at that specific moment.
Politicians know that, but it would be political suicide to go out publicly and tell it like it is. Crime prevention programs, correctional facilities, police, psychiatric systems, urban housing projects, etc. all need some major attention and funds to stem the recent development in crime.
Bu that costs money and no politician is going out there asking for money, when its much cheaper to pass a bill, making you appear as a man of action.