The reason OxyContin became popular for recreational use is accessibility. As HongPong hinted at in his writeup, OxyContin first registered as a serious problem in places like eastern Kentucky, West Virginia and southwestern Virginia. OxyContin users, as opposed to users of other illicit drugs, did not have to access underground supply chains (in the case of imported drugs) or grow it (marijuana -- this is also a prime marijuana-growing region of the US). They could either steal it from the local pharmacy or simply get a complicit doctor to prescribe it unnecessarily.

Combine this accessibility with the sad economic shape of the Appalachian region, and you had the rise of a new drug, which first became known widely as "the hillbilly heroin." It did not take much money or effort to get into this habit, and, as other WUs have mentioned, it's incredibly difficult to break.