For all we know, this could have been leaked by Joe the North Korea analyst to raise his profile and increase his status.
- NK is developing longer-range ICBMs, and relations with the US have been deteriorating; recently, in response to a Trump tweet, NK threatened to drop a nuke on NYC [1]. Nobody in the administration is going to complain if NK takes a propaganda hit.
- The NSA has been taking a lot of blame for WannaCry, and pointing fingers at NK diverts some of that blame.
[1] http://www.newsweek.com/north-korea-attack-new-york-nuclear-...
Which is the exact reason I wouldn't trust any NSA-provided news about the origins of WannaCry more than I trust an average warez-site.
Budget. If they can spin this as a national security matter rather than a criminal matter they get the money rather than the FBI cybercrimes division.
NK specifically has the advantage that nobody will believe them and there is no political fallout from accusing them.
I'de say it's highly likely too, but what we need to find out is why would the US deep-state do this? Is there a specific motive for the US deep-state haphazardly implicating NK, other than, just maybe, the NSA got their hands on some evidence that we aren't aware of?