The United States don't need "excuses" to attack anyone (conventionally or with WMD). They need only opportunity to attack somebody with profit and without retaliation. North Korea have good deterrence on the USA and can be quite provocative. If the USA nuke DPRK, DPRK nuke the USA back and destroy, say, five big cities.
So, if there is a choice between swallowing nuclearisation of a previously non-nuclear country (as it was with India, Pakistan, Israel, North Korea) and fighting and even winning a limited nuclear war - latter is usually lesser acceptable, especially if even a limited nuclear war means that many more previously non-nuclear countries became nuclear in few years.
And, of course, American illegal and unprovoked aggression against Better Korea will face Russia and Chinese response.