There are definitely economies of scale when it comes to farming and weed is no exception. That, and taking advantage of cheaper labor costs elsewhere (like Mexico)

That, and it may be easier to grow in a laxer jurisdiction and transport it to a stricter one, than just trying to grow it in a strict one.

Salad grows pretty much everywhere, but we still transport most of it from Mexico or California, with a pretty hefty set of infrastructure components from farms to hordes of pickers to refridgerated trains to grocery stores.

(It's also not a weed, really, just like it's not a pot.)