Think Brad Stevens knows he's not a good fit for the team on the court and contract wise. He's coached him and seen how he's done within the system and now he's overseeing the re-build of the team.
Kemba was the big fish in the small pond in Charlotte, and he's arguably taken a back seat to Tatum and Brown (who apparently have some issues between themselves too). And he's been injured the past couple of seasons.
Bringing back Horford is about leadership and a constant guy to lean on. Then his contract is out. More cap space in 2022. And as
@JPRouve mentioned, there's a good possibility another move or two are in the works to acquire a point guard.