Oxycontin can be attributed slightly to this... As it's popularity grew, so did the price. Good kids and normal people became addicts. They soon found heroin is cheaper, and is fairly easy to get in Indy, L'ville, or Cincinnati.
Where there is an opening in a black market, most likely some criminal will fill it.
When Purdue Pharma changed their formulation of Oxycontin pills to a nearly abuse proof formula, one you couldn't easily crush and abuse, this also helped increase the attractiveness of H.
And if you think this is a recent development, don't be so naive. I lived in Seymour for 6 years of my 20's and happened to know one guy that is doing a 10 year sentence for bringing in large amounts of H from Indy.