Tony Dokoupil explains to Terry Gross:

So in Colorado, the medical marijuana growers have to have 24-hour video of their operations, and that video is accessible by the state at any time — they can tap into it. And they all have these badges, and they had to go through background checks to get these badges. So there's an attempt to keep black market money out of it ... [I]n Colorado, you're unlikely to see a situation where a dispensary is, in fact, just a front for a Mexican cartel. And then they had this additional rule, which is pretty revolutionary — it's called a 70-30 rule, where 70 percent of all the marijuana that each store sells, they have to grow themselves. ... That goes a long way to eliminating the introduction of black market weed. ...

That's a huge reason why the feds have focused on California.