The pulled bill was the result of two different forces coming into play: Pro-IP monetization companies applying pressure and coalition splitting among the Democratic caucus.
The IP Monetization folks (Trolls, Universities, Bio, PHrMA, old line industry, and folks like Qualcomm) have been pushing really hard to weaken the legislation. There were substantial efforts to come to reasonable compromises, but those groups were never really happy.
What ultimately killed it, though, is that there were some important special interest groups (notably the trial lawyers) who opposed any kind of reform. If the bill would have been brought to the floor, it would have split the Democratic coalition and made a bunch of big-money donors mad.
When the opposition from the IP Monetization group mixed with the destabilizing political effect, the Senate Democratic leadership decided to kill it.