Same reason they got involved in the first place - $$$$$$$.

That said, the Mafia's involement in the drug trade is limited today. Having been marginalized over the past 25 years or so, they are no longer the dominant players. In fact, looking at the remaining viable families, only the New York families really have significant drug operations. But even there they are more at the mid to lower levels. Other families, including New England, New Jersey, Philadelphia, Chicago, and Detroit, have relatively little involvement in narcotics.