The second best home run hitter was up to bat next. On a percentage basis for a single at bat, the chances of either batter hitting a home run is pretty close to the same, but if you walk the first batter, that just means there is an extra runner on base to score if batter number 2 then hits a home run.
The coaches have pages and pages of stats and tendencies to consider when making these decisions, not just the single data point you are referring to.
It was also a "missed pitch" by Mancha, who typically keeps the ball low and on the corners.