Some regulators are already looking into it. Even at this point you'd either need multinational cooperation or you'd need China to decide that 51% attacking a budding technology is a good thing to do, something that would be sure to increase tensions across the world.

But there are two bigger reasons. The first is that regulators are used to doing regulation at exchange points, regulating mining is new and unfamiliar and requires a decent understanding of blockchains. And the second is that Bitcoin is tiny potatoes at this point. To the best of my knowledge, organized crime outside of DNMs doesn't use Bitcoin. There's minimal reason to target it while it's so small.

Regulated mining I believe is going to be a genuine risk as Bitcoin grows.