Symantec attributes the decline to the halt in spam-sending activities from a trio of botnets – Rustock, Lethic and Xavester. Another contributing factor was the shuttering of SpamIt, one of the largest drug-spamming affiliates. Still, according to the report, spam still seems to be working:
For now, it's clear that spam remains a profitable activity for spammers and there is no evidence to suggest that pharmaceutical spam is suffering any loss in profitability ... However, the closure of SpamIt re-shaped the activities of many of the major pharmaceutical spam gangs as pharmaceutical spam operations underwent restructuring and consolidation.