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?

Snowden.

Much more likely the bad reaction to Eternal Blue.

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.

You write that like it's a bad thing.

You can do the right thing for the wrong reasons.

An alternative angle that could make sense is that it shows that they're not purely intent on hoarding exploits (particularly dangerous ones) and are willing to report them to software vendors in order to reduce everyone's risk profile.

That'd be more of a communal-good, de-escalation approach. There's certainly something to be said for the fact that it displays the talent and expertise available too though (i.e. helping for recruitment).