In Federal courts mandatory minimum sentences were judged to be unconstitutional, as the ability to individualize sentencing was considered a prerogative intrinsic to the role of [Federal] judges. Though, a judge cannot impose a sentence greater than the maximum allowed under law. Federal courts still have sentencing guidelines that are almost always applied, but strictly speaking they're advisory.

More fundamentally, individualized justice is a core principle of common law courts, at least historically speaking. It's also an obscure principle, but you can't fully understand the system without it, including the wide latitude judges often wield in various (albeit usually highly technical) aspects of their job.