Since the advent of the Sentencing Reform Act of 1984, imprisonment has been the dominant sanction in the federal criminal justice system. In 2014, 90% of defendants convicted of federal crimes went to prison whereas 50% went to the pokey in 1980. Federal courts sentenced 2,300 fewer offenders to probation in 2014 than in 1980, even though their criminal caseload nearly tripled during that time.
Changes in the kinds of offenses prosecuted in federal court have undoubtedly contributed to the shift toward prison and away from probation. But, sentencing policies established during the 1980s and 1990s have also played an important role by mandating prison time for many offenses for which probation had been ordered in the past.
During the 1980s and 1990s, lawmakers frequently enacted laws prohibiting the imposition of terms of probation and requiring prison terms for many federal crimes, including drug trafficking and illegal firearms possession. To compound this, in 1984, Congress created the Sentencing Commission and charged it with establishing guidelines that federal judges were required to follow during sentencing. The guidelines, which were intended to promote consistency in federal criminal penalties and took effect in November 1987, mandated imprisonment for a variety of offenses (fraud, embezzlement, and tax evasion) for which probation was once a routine sanction.
http://www.pewtrusts.org/~/media/assets/2016/01/pspp_fs_moreprisonlessprobation_v1.pdf