That’s because:
1) He’s been a zero-chance candidate for weeks
2) The contest is obviously between Macri and Scioli
3) Massa moving to become a candidate for governor inside the Macri campaign simply formalizes the split that his Frente Renovador has been going through recently, with some of his members bound to follow him, others that will move to Scioli.
It’s been a de-facto two horse race since around April. All that changes is the removal of “de-facto” from that sentence.
UPDATE: The three charts in this post from José Rubén Santís today, all about a brand new poll this morning (that came after this IKN post’s publication, fwiw) show just why the above is true. The post is in Spanish but it’s easy enough to understand the chart information. And again, for those who want deeper insight into the delcine and fall of Sergio Massa and what it’s all leading towards, this post over at Los Huevos y Las Ideas dated June 2nd to which IKN previously linked does a fine job. Suitable for those adept to the tongue of Cervantes.