The history of mandatory sentencing is tied to slavery.

"Congress also used mandatory minimum penalties in its efforts to end the importation of slaves. The Constitution prohibited Congress from curtailing or abolishing the importation of slaves before 1808. In advance of the 1808 date, and with President Thomas Jefferson’s urging, Congress passed an Act prohibiting the importation of slaves in February 1807. Among other provisions, the 1807 Act prohibited citizens from bringing slaves into the United States or serving on a vessel that transported slaves. These offenses carried mandatory minimum penalties of at least five years and two years of imprisonment, respectively. However, the mandatory minimum penalties were much less severe than the mandatory death penalty many in the House of Representatives wanted to attach to these offenses, on grounds that importing human beings was a crime of morality and akin to murder. Other offenses created by the 1807 Act, such as outfitting slave vessels and purchasing or selling illegally imported slaves, carried only fines.”

 http://www.ussc.gov/sites/default/files/pdf/news/congressional-testimony-and-reports/mandatory-minimum-penalties/20111031-rtc-pdf/Chapter_02.pdf