I'll be real, the only reason we haven't is because there is worry they already have the bomb, and disguised it as an a NK nuke test.
What I was told is that one of the nuke sniffers planes found an anomalous isotope ratio in on of the tests, distinct from the other NK tests, and the US goes to a lot of expense to know which mine and which reactor any fissile material came from.
If Iran had done this, and had successfully tested a nuke in NK, it would explain a lot of the moves DC has made in regards to Iran over the past decade or so, particularly the Iran Nuke Deal and why Iran has been allowed to get away with so much shit.