If you believe people aren't acting rationally based on the information you have, shouldn't you first question your assumptions?
My own take would be that Al Qaeda is (generally) forced to import plotters, builders and attackers whereas the drug trade uses locals and thus enjoys innumerable advantages.
Simply: 10 lifers in a couple trailers in the back woods of Kentucky, buying, say, a few guns and cleaning supplies in bulk doesn't raise the same flags as 10 foreign guys, new to the area, with funny clothes and accents.
Further, there are plenty of people who know the tweakers are out there cooking meth or the local gang is pushing drugs. But they're such an ingrained regular presence, these people have normalized it into their life.
In any event, if it truly were easier to get 10 guys with guns to open fire in the US, I'd expect we'd have seen it by now.