Democratic Primaries

Of the two points brought up:

1. Biden didn't support "overhauling" Social Security, but make relatively minor changes to it - changes that from time to time even Sanders advocated.

2. Although Biden did author the bill, Sanders (in the House at the time) voted for the bill.

It could be that they didn't "go negative" because that could have backfired considering Sanders' record on both issues.

Parenthetically, after the bill was passed it was noted that the penalties for the two types of cocaine were inconsistent. Accordingly Democrats in 1995 proposed to lessen the penalty for crack cocaine to that of powder cocaine. Sanders balked at that - instead he wanted to RAISE the penalty for powder cocaine to that of crack cocaine, effectively strengthening the penalties in the bill that Democrats wanted to weaken.

This may be another reason why the Sanders campaign didn't want to "go negative" - it would have exposed glaring faults in his positions on the 1994 Crime Bill.