Because Ben Brown was the only pitcher who was well rested and had success against the Brewers this season, which seemed to be your criteria.
I get the criticism, but all of the options came with big question marks. They picked wrong and adjusted quickly. Unfortunately Soroka was worse. Here is the truth with this series, the Cubs are in a bad way with starting pitching. They are going to need to score 5+ runs a game to have a chance.
A healthy Horton or close the Padres out in 2 and it would not have been so bad