These people deserve to die as they have ruined many lives and there is no place for them in society.
If it isn't mandatory then judges and juries will be too lenient and they will get away with their crimes. It is the only way to stop more crimes being committed.
A Texas teen recently got 10 years of probation for killing 4 people while drunk. The judge gave the juvenile leniency because he believed a rogue psychologist who said the young man had "affluenza" due to his rich upbringing. Victims' families were outraged he wasn't given jail time. Cases like these should only be given leniency in extreme cases. Mandatory sentences should be carried out unless the law specifically states what "extenuating circumstances" allow for lighter sentencing guidelines.
I do not believe that there is popular pressure for mandatory sentencing, because the popular opinion in society is that prison terms are to extensive in general. When you take away the possibility of parole, or early release it becomes inhumane. Especially when most crimes committed are not serious enough for extended jail, or prison time.