Legislators respond with the tools that they have to public concerns and to troubling events. One can see, in the list of major criminal law enactments in this post, the history of punitive responses to recurring waves of legitimate public concern about drunk driving, drugs, guns, domestic violence, youth violence, sex offenses and crimes against the elderly.
Incarceration in Massachusetts started to climb after crime surged.
From the early 1960s through about 1990, crime rose dramatically in Massachusetts. Most categories of crime have fallen since the early 1990s, but remain above their level in 1965. However, the state prison population has remained elevated, as the chart in this post (indexed to 1965 levels) demonstrates.