Drugs in Northern New Mexico

The New York Times profiles the county of Rio Arriba, just north of Santa Fe.  This county, which includes Espanola, has the highest incidence of heroin overdoses in the country.  Espanola was also known to me as the closest Walmart and large movie theater to Los Alamos.

From the article, it seems like everyone is addicted to heroin in this county - grandparents down to teenagers.  What makes me curious is why heroin seems to be the drug of choice in this county rather than methamphetamine?  I have heard stories that this town is on the prime route for moving drugs from Mexico into middle America, but one would guess that meth should be just as easily obtained from Mexico as heroin. 

Tradition?  Availability?  Social taboos?  I don't really know.