I'm not sure if you have experience with the justice system but sentencing has 5 factors that judges take into consideration, this individual has a low chance of recidivism and I bet his COMPAS report when he's assessed in federal custody will affirm this.
You're comparing apples to oranges, a sexual offender vs an offender who killed someone while drunk is not a comparison. The general rule of thumb for those convicted of sexual assault is 3-4 years (look into case law and you'll see) and considering this offender has a higher chance of rehabilitation (his letters of support from his church rehabilitation for his previous crimes) his sentence is within the realm of similar crimes. If anything a longer sentence would actually increase his chance of recidivism.