Bailey chose to come here as a pitching coach rather than other bench opportunities that in theory would better further is career.
I don't know the internal machinations going on on Yawkey way, but I'd argue that Bailey - obviously a bright guy - could have taken a look at the staff/farm here and seen a better opportunity to solidify his resume by turning around the pitching staff of a - still - highly regarded, cornerstone franchise in the MLB. Early results look spectacular and I'm sure he's right on top of many team's lists at the moment. Also, he likely understands that the better he is at his job the likelier it is that Cora gets extended. If this team overachieves and makes the playoffs, I'd say it's a lock that Cora gets extended and also likely that Bailey moves on to take the big seat elsewhere. It honestly wouldn't surprise me if Bailey's ultimate goal is an executive role.
It's funny how in the fandom Cora ranges from Co-GM to lame duck keeping the seat warm for his pitching coaching who's gotten a wink and a nod about how his future in Boston might play out. This will be fascinating to track.