Or maybe because there wasn't actually a good case to block the merger? Neither Activision-Blizzard nor Microsoft are anywhere near monopolies in gaming. Believe it or not "big tech is bad" is not a sufficient case for blocking a merger, actual consumer harm or threat of it has to be demonstrated.
I'm less convinced this is an issue of Kahn and her team's inexperience, so much as that the FTC is being expected (or maybe directed by the executive) to take on cases that don't have any merit to placate public opinion. "Big tech bad" is a popular meme these days, and the FTC is pursuing cases accordingly even if those cases are on very shaky ground.