Back in the 80s crime was rampant and rising. Drugs were driving the increase. Lawmakers looked at the situation and noticed that drug dealers were a VERY prolific group of criminals. They not only sell drugs, they commit assault and murder to protect their territory. They engage in loan-sharking. They run prostitutes. They hire enforcers to collect debts and encourage their prostitutes and debtors to pay up. They fence stolen goods which they receive in exchange for drugs. Etc. etc. 
Lawmakers learned that about 13% of criminals were committing nearly 90% of the crimes and a very large percentage of those especially prolific offenders were dealing drugs. What the largely liberal Democrat promoters of sentence reduction fail to notice is that putting these high output thugs away for VERY long sentences has WORKED. There is no such thing as a non-violent, first time drug offender with a long sentence. It's a myth and this will not turn out well.