They have probably done that for a while (this is the first public attribution, not the first disclosure); but they are now blowing their trumpet because they need some good PR. Why?
EternalBlue would have not received that much coverage had it not happened after Snowden proved that the American public cannot trust the agency. They had been dragged to the foreground before without repercussions, because reactions were limited to the IT world. Snowden made it a general-public issue, and now they are forced to to shape up.
Snowden.