Scioli, the progressive, two-term Governor of Buenos Aires Province (at 16 million people, the most important in Argentina by far), wasn't endorsed by the President as much as he was by the polls themselves - which gave him a 3-to-1 advantage over Randazzo in the primaries.
Randazzo had several strengths: he's successfully managed the renationalization and refurbishment since 2013 of Argentina's overworked (and, since being privatized in 1992, overbilked) railways; he was the favorite of the party's left wing and its influential youth chapter, La Cámpora; his affable manner appeals to many voters; and most importantly, none of attacks the "Fox news of Argentina" (the Clarín Group) has thrown at him has stuck.
He was, however, less appealing to potential swing voters (mainly because they knew the left preferred him) - especially to working-class voters, who prefer the more assertive Scioli. And Randazzo's a confirmed bachelor at age 51 - a subtle but significant drawback in this mostly Catholic country. He's been doing a great job in his current post, and if Scioli wins my hope is that Randazzo can stay on as Interior and Transport Minister for at least two more years.