Because its still an individual game... with individual perceptions of pressure. Carter has been out of the lineup for much of the second half/slide... he really wasn't directly dealing with the day-to-day ebb/flow and some of the recent crushing losses.
mccullers had a real short leash this inning and is rightfully getting yanked. can't give up a walk to the lead off hitter.
Jake might have more experience (esp recently) in the corners than Gomez? Given that they're both elite in CF, the minimal difference in CF might be outweighed by comfort in the corners